Cosmic Dawn at High Latitudes Conference

24 Jun 2024, 09:00 → 28 Jun 2024, 17:00 Europe/Stockholm

Beijer auditorium (Swedish Royal Academy of Sciences)

Background

The Cosmic Dawn is the era when the first stars and galaxies formed and which set in motion a series of fundamental changes in our Universe. This Nordita program is dedicated to studies of these early galaxies and how they changed the matter between them, the intergalactic medium, from cold and neutral to hot and ionized in a process called reionization. The program considers both the properties of the galaxies and the properties of the intergalactic medium. This allows us to address how on the one hand studies of galaxies can answer questions about the evolution of the intergalactic medium, both before and during reionization, and how on the other hand studies of the intergalactic medium can improve our understanding of the formation and evolution of galaxies. In this context, the program will address the implications of the latest observations, such as by the James Webb Space Telescope (JWST) and various 21-cm experiments, expectations for future observations, as well as the latest developments in modelling the processes during the Cosmic Dawn and the use of these in interpreting the observational results.

 

The conference is part of a Nordita program with the same title and similar to the program will focus on the following four themes:

1. When & how did reionization take place? Evolution of the mean ionized fraction; evolution of ionized bubble sizes; evolution of neutral islands; measuring these quantities.   
2. Sources & sinks of reionization: Production & escape of ionizing photons; absorption of ionizing photons in the intergalactic medium; absorption of ionizing photons at large and small scales.   
3. Physical properties of high redshift galaxies. What have we learned from JWST, ALMA, etc.? What can we learn from low redshift analogues? What do simulations tell us?   
4. What happened before reionization really started? Early star formation; Pop III stars; early X-ray sources; cosmology

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Important dates

Conference abstract submission deadline - March 11th  
Abstract assessment - by March 29th 
Applications/Registration deadline - April 14th 
Applications assessment results, including financial support decisions - by April 30th 
Individual accommodation confirmations to relevant participants - by May 14th 
Welcome letter with practical information and directions - by June 5th

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Relation Program and Conference

This conference is embedded in a Nordita program which has its own website. However, registration is shared between the program and the conference. We encourage conference attendees to become program participants in either the weeks before or the week after the conference. If you only want to attend the conference, please select week 3 when you register. 

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Invited speakers

• Benedetta Ciardi
• Danielle Berg
• Frederick Davies
• Yuichi Harikane
• Laura Keating
• Joakim Rosdahl
• Dan Stark

 

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Organizers:

Sambit Giri, Matthew Hayes, Anne Hutter, Charlotte Mason, Garrelt Mellema, Göran Östlin, Sune Toft

 

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SOC

Sarah Bosman  
Sambit Giri
Matthew Hayes  
Anne Hutter   
Anne Jaskot  
Matt McQuinn  
Charlotte Mason   
Garrelt Mellema  
Desika Narayanan  
Renske Smit  
Cathryn Trott  
Naoki Yoshida

Sponsored by:

Royal Swedish Academy of Sciences, Wenner-Gren Foundations, Nordita

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Monday, 24 June

09:00 → 09:30 Welcome and practical information

Speaker: Garrelt Mellema (Stockholm University)

09:30 → 12:30 Session 1

Convener: Andrei Mesinger

09:30 Charting the progress of reionization with quasars and galaxies

Speaker: Laura Keating

Keating_Stockholm_24june.pdf

10:00 Simulating Cosmic Reionization on all scales

Simulations of the early structure formation and the Epoch of Reionization have
now reached sufficient volume, dynamic range and resolution to make reliable
predictions of the fundamental features and observable signatures of these
epochs at the full range of relevant scales. I will summarise important recent
progress our group has made in this area based on an ongoing, multi-year
simulation project. It includes performing a series of structure formation,
radiative transfer and radiative hydrodynamics simulations of early structure
and galaxy formation on all relevant scales, from the tiny cosmological minihaloes
hosting the very first stars up to very large volumes of hundreds of Mpc.

A highlight of this project is the Cosmic Dawn suite of fully-coupled N-body
and radiative hydrodynamics simulations, which by some measures is the largest
such simulations ever performed, with up to 8192^3 (512 billion) particles and
8192^3 radiative transfer and hydrodynamics grid. These flagship simulations are
complemented by highly-zoomed simulations of the detailed small-scale structures
and feedback, particularly in the context of our own Local Group, as well as by
very large-scale volumes, matched to the full fields of view of redshifted 21-cm
experiments like LOFAR ans SKA. Furthermore, I will present the first implementation
of the effects of baryon-dark matter drift velocities within large-volume zoom
simulations of early structures.

Speaker: Ilian Iliev (University of Sussex)

10:15 Cosmic Dawn III: balancing ionizing photon production and the impact of dust during the Epoch of Reionization

Cosmic Dawn III (CoDa III) is a fully-coupled radiation-hydrodynamics simulation of cosmic reionization and galaxy formation and their mutual impact, to redshift z < 5. CoDaIII was completed on Summit (Oak Ridge Leadership Computing Facility), and features an improved resolution in mass, 8 times higher compared to our previous iteration CoDa II. With 8192^3 particles and cells in a (94 Mpc)^3 box, it is large enough to model global reionization and its feedback on galaxy formation while resolving all haloes above 2.10^7 Msun. To accomplish this massive numerical enterprise, CoDa III uses the hybrid CPU-GPU code RAMSES-CUDATON (Ocvirk et al. 2016, 2020), deployed on 24576 GPUs and 131072 CPUS, making it the largest simulation of the Epoch of Reionization ever performed, with about 20 PetaBytes of data produced, as well as the most massively parallel setup ever achieved with RAMSES. CoDa III modified and re-calibrated the subgrid star-formation algorithm with respect to our previous simulation CoDa II, making reionization end slightly later, at z ~ 5.5, dramatically improving the agreement with the observations of intergalactic Lyman-alpha forest in quasar spectra, in both opacity and ionizing rate evolution, while conserving the good match already obtained with electron-scattering optical depth from cosmic microwave background fluctuations. I will show how this improved agreement came to be by examining the respective properties of the intergalactic medium and the galaxy population in the simulation, and in particular how external radiative feedback is a crucial ingredient. Finally, I will also highlight the impact of dust on ionizing photon escape in the simulation, and confront our predictions with recent observational constraints from e.g. JWST. From there, I will discuss remaining challenges in the analysis of such simulations and the next important evolutions in the field.

Speaker: Pierre OCVIRK (universite de Strasbourg)

10:30 A semi-analytic model for Pop III star formation

The formation of the first stars marks a watershed moment in the history of our universe. As the first luminous structures, these stars (also known as Population III, or Pop III stars) seed the first galaxies and begin the process of reionization, but are challenging to observe. We construct an analytic model to self-consistently trace the formation of Pop III stars inside minihalos in the presence of the fluctuating ultraviolet background, relic dark matter-baryon relative velocities from the early universe, and an X-ray background, which largely work to suppress cooling of gas and delay the formation of this first generation of stars. We demonstrate the utility of this framework in a semi-analytic model for early star formation that follows the transition between Pop III and Pop II star formation inside these halos and computes the radiation backgrounds that will determine the 21-cm spin temperature. Using our new prescription for the criteria allowing Pop III star formation, we follow a population of dark matter halos from z = 50 through z = 6 and examine the global star formation history, finding that each process defines its own key epoch: (i) the stream velocity dominates at the highest redshifts (z>30), (ii) the UV background sets the tone at intermediate times (30<z<15), and (iii) X-rays control the end of Pop III star formation at the latest times (z<15). In all of our models, Pop III stars continue to form down to z ∼ 7 − 10. In this talk, I will present our semi-analytic model and detail how it can be leveraged to predict key observables—such as for transient surveys and 21-cm and line intensity mapping experiments—that can help to constrain the properties of this first generation of stars.

Speaker: Sahil Hegde

10:45 Anaxagoras: Towards ab initio predictions for the First Stars and Galaxies

The first stars, star clusters, and galaxies are expected to have formed in starbursts within low-mass halos at Cosmic Dawn. To test our understanding of Cosmic Dawn and ΛCDM, accurate predictions of their properties are crucial, especially as new high-redshift observations begin to emerge. Current cosmological simulations, are typically numerically expensive and lack either sub-parsec resolution or a realistic implementation of stellar feedback. I will present preliminary results from Anaxagoras, a novel semi-analytical model of starbursts at Cosmic Dawn. Unlike existing semi-analytical models, Anaxagoras adopts an ab initio approach, incorporating every major stellar feedback process without fine-tuning. While physically comprehensive, Anaxagoras is fast enough to run on a laptop. I will discuss the model's predictions for the properties of the first Pop III and Pop II star clusters and galaxies.

Speaker: Olof Petter Nils Axel Nebrin (Stockholm University)

11:00 Coffee break

11:30 COLDSim: A new look at H$_2$, H I and H II in cosmological simulations.

The latest ALMA and JWST observations provide new information on the formation
and evolution of galaxies in the early Universe, at the Epoch of Reionization. Of
particular importance are measurements of the molecular gas budget of these z >
5 objects, which is known to be the main fuel for star formation. A powerful
tool for measuring the gas content in galaxies at the Epoch of Reionization is the
[C II] emission line. Due to its low excitation potential, [C II] emission can be
produced in photodissociation regions, neutral atomic gas and molecular clouds. To
properly capture the cold-gas processes taking place in such environments (molecule
formation, self-shielding, dust grain catalysis, photoelectric and cosmic-ray heating),
we introduce a new set of state-of-the-art hydrodynamic simulations (COLDSim)
including time-dependent non-equilibrium chemistry, star formation, stellar evolution,
metal spreading and feedback mechanisms. In this way, we are able to accurately
track the evolution of H I, H II and H$_2$ in a cosmological context and predict the
contribution of each gas phase to high-redshift [C II] luminosities. We also show
how [C II] luminosities correlate with SFR and stellar mass of the hosting galaxy,
and how these relations evolve with redshift. Moreover, we probe the correlation
between [C II] luminosity and molecular gas mass and provide theoretical values
for the conversion factor $α_{[CII]}$ from z = 6 to z = 12. These physically-motivated
predictions can be used in observational works to infer the molecular budget at
primordial epochs.

Speaker: Benedetta Casavecchia (Max Planck Institute for Astrophysics)

Casavecchia_CosmicDawn.pdf

11:45 The challenging and promising path towards accurate predictions of the EoR clustering signal

The 21cm signal from the Cosmic Dawn and the EoR contains extremely valuable information about many aspects of cosmology and astrophysics that cannot easily be obtained by other probes. I will start my talk by summarising our recent attempts to improve the modelling of the 21cm clustering signal, showing that we (and others) are still far away from the precision requirements of SKA-Low. In the second part, I will show new results on cosmology, dark matter physics, and the prospects to detect intermediate-mass black holes at cosmic dawn.

Speaker: Aurel Schneider (Department of Astrophysics, University of Zurich)

12:00 First stars and binaries

In this work, we incorporate the stochastic nature of X-ray sources into the 21-cm signal simulations of Cosmic Dawn. Traditionally, simulations of the 21-cm line have assumed a constant or smoothly varying X-ray background. This simplicity, however, ignores the intrinsically stochastic distribution of X-ray sources, including X-ray binaries, which are believed to have a major impact on the Intergalactic medium (IGM) temperature and ionization history. We intend to capture the intricate relationship between X-ray luminosity, source distribution, and the heating of the IGM by incorporating a stochastic model for X-ray source distribution into current 21-cm simulations. According to preliminary findings, the predicted 21-cm signal exhibits alterations when X-ray stochasticity is introduced, particularly with regard to a more complicated and heterogeneous thermal history and ionization than previously thought. It holds the potential for novel observational signatures that could be targeted by upcoming low-frequency radio telescopes such as the Square Kilometre Array (SKA).

Speaker: Anastasia Fialkov (Institute of Astronomy, University of Cambridge)

12:15 Exploring Beyond Standard Cosmological Models during the Epoch of Reionization

The epoch of reionization marks the emergence of the first galaxies that emit light into the intergalactic medium, leading to the ionisation and heating of the Universe. This phase is at the forefront of astrophysics, serving as the era where the 'seeds' for modern-day galaxies were formed. While our current understanding relies on theoretical models of early galaxy formation within the standard cold dark matter (CDM) cosmology, recent observations, including early galaxy data from the James Webb Space Telescope (JWST) and the thermal evolution of intergalactic medium from the Experiment to Detect the Global EoR Signature (EDGES), have raised questions about this conventional view. In this talk, I will discuss the insights gained from current observations regarding the nature of dark matter and dark energy, as well as the future prospects for refining these constraints. Additionally, I will explore the potential of the Square Kilometre Array (SKA) telescope, a more powerful instrument poised to observe the intergalactic medium during these early cosmic epochs. I will present a forecast study on the capability of SKA to study cosmological models, shedding light on its role in advancing our understanding of the universe.

Speaker: Sambit Kumar Giri (Stockholm University, Nordita)

CosmicDawnAtHighLatitudes_2024.pdf

12:30 → 13:45 Buffet lunch

13:45 → 17:30 Session 2

Convener: Laura Pentericci

13:45 The role of clumping and photon sinks during reionization

Speaker: Fredrick Davies

14:15 What is JWST telling us about cosmology?

The launch of the James Webb Space Telescope (JWST) has ignited a revolution in our understanding of the early universe. Its exquisite infrared capabilities have allowed observers to measure the stellar masses of galaxies, as well as to find them at higher redshifts than ever before. I will describe how, intriguingly, observations in these two arenas appear to be in tension with our models. First, I will discuss the ultra-massive galaxy candidates recently argued to “break” our cosmological model (LCDM). I will show how HST observations at the same redshifts rule out the required departures from LCDM, disfavoring a cosmological explanation of their abundances. I will also briefly discuss the higher-than-expected abundance of z>10 galaxies in JWST, and how clustering measurements can break degeneracies in our models and inform the pathway to form the first galaxies.

Speaker: Julian Munoz (UT Austin)

14:30 The Role of Galaxies and AGN during Reionization: Insights from JWST ASPIRE Quasar Fields and Subaru IGM Tomography

Mapping the large-scale structures of galaxy distribution and the intergalactic medium (IGM) — 'the art of cosmography' — has been instrumental in advancing our understanding of the Universe. The James Webb Space Telescope (JWST) has revolutionized the study of distant galaxies and cosmic reionization. However, the entire structure of the 'cosmic web' still remains unseen. In this talk, I will present our results for mapping the entire cosmic web, including both galaxies and the IGM, from the JWST ASPIRE quasar field survey. We find evidence of spatial correlation between JWST OIII emitters and the surrounding IGM transmission in the Lyman-alpha forest in multiple quasar fields. OIII emitters are found in the vicinity of IGM transmission spikes at 5.4 < z < 6.5, indicating that they reside in the highly ionized environment of the IGM and/or ionized bubbles. By comparing the observed cross-correlation with THESAN cosmological radiation hydrodynamic simulations, we find an excellent agreement between observation and simulation, reinforcing the idea that galaxies are the main driver of reionization. We also discuss the relative roles of luminous and faint galaxies by examining the individual associations between OIII emitters and transmission spikes. I will conclude by discussing the prospect of mapping the entire cosmic web during the final stages of reionization through photometric IGM tomography with JWST, Subaru/Hyper-SuprimeCam+ Prime-Focus Spectrograph, and Euclid in the near future to better understand the role of galaxies and AGN during the reionization epoch.

Speaker: Koki Kakiichi (Cosmic Dawn Center, University of Copenhagen)

14:45 Galaxies as Agents of Cosmic Reionization: Insights from the EIGER Survey

We present the results from the EIGER survey, conducting JWST/NIRCam WFSS surveys in the fields of luminous quasars at $z \ge 6$. Our analysis includes observations from five of the six designated quasar fields, leading to the identification of approximately 750 [OIII]-emitting galaxies with spectroscopic confirmation across a redshift span of $z=5.3$-–7.0. By examining both the spatial distribution of these galaxies and the spectral data from the associated quasars, we have pinpointed specific instances of co-location of galaxies and ionized regions, as traced by Lyman-alpha and/or Lyman-beta transmission spikes. This correlation serves as direct proof of galaxies facilitating localized reionization events. Additionally, our findings reveal that, at lower redshifts ($z<5.7$), approaching the end of reionization, transmission is more suppressed nearer to galaxies within $\sim10$ cMpc, likely reflecting overdensity around them. Conversely, at higher redshifts, excess transmission is observed $\sim 7$ cMpc away from galaxies. All these observations align with the 'inside-out' scenario that reionization occurred from overdense regions where galaxies began forming earlier.

Speaker: Daichi Kashino (NAOJ)

kashino.pdf

15:00 JWST PRIMER: A new multi-field determination of the evolving galaxy UV luminosity function at redshifts z≃9−15

I will present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range $8.58.5)>0.05$) to undertake a statistical calculation of the evolving UV LF. Our new measurements span $\simeq4$ magnitudes in UV luminosity at $z=9-12.5$, placing new constraints on both the shape and evolution of the LF at early times. We fit our observational data-points with a double-power law (DPL) function and explore the evolution of the DPL parameters. Our UV LF measurements yield a new estimate of the early evolution of cosmic star-formation rate density ($\rho_{\rm{SFR}}$) which confirms the gradual, log-linear decline deduced from early JWST studies, at least out to $z \simeq 12$. Finally I will show that the observed early evolution of the galaxy UV LF (and $\rho_{\rm{SFR}}$) can be reproduced in a ${\rm \Lambda}$CDM Universe, with no change in dust properties or star-formation efficiency required out to $z \simeq 12$. Instead, we find that a progressive trend towards younger stellar population ages can reproduce the observations, and we show that the typical ages required at $z \simeq$ 8, 9, 10, and 11 all converge on a time $\simeq 380-330$\,Myr after the Big Bang, indicative of a rapid emergence of early galaxies at $z \simeq 12 - 13$. This is consistent with the first indications of a steeper drop-off in the galaxy population we find beyond $z \simeq 13$, possibly reflecting the rapid evolution of the halo mass function at earlier times.

Speaker: Callum Donnan (University of Edinburgh)

15:15 Afternoon coffee / tea break

15:45 Distinguishing reionization scenarios in the JWST era and improvements in IGM modeling

It has been noted recently that the latest JWST observations at z > 6 may suggest ionizing photon production by galaxies significantly in excess of previous expectations. Taken at face value, this apparent excess implies an earlier end to reionization (at z ~ 8) than suggested by measurements of the CMB optical depth and especially the 5 < z < 6 Ly-alpha forest. There are two viable avenues for re-establishing agreement with forest observations, both of which require reionization to end at z < 6. Either reionization began relatively late (at z ~ 8-9), proceeded rapidly, or it began early (z > 12) and ended gradually. I will discuss the implications of these two scenarios for the evolution of the ionizing properties of galaxies, in light of the latest measurements of the UV luminosity function from JWST. I will also explore the concordance of these models with QSO observations at 5 < z < 6 and observations of bright Ly-alpha emitters at z > 8, with a focus on whether the late-starting model can explain the latter. If there is time, I will highlight recent developments in my work to improve the efficiency and accuracy of reionization simulations. These include a new suite of multi-scale, high-resolution hydro/RT simulations of IGM gas dynamics spanning a wide parameter space of large-scale IGM environments and reaching sub-kpc resolution. These simulations will allow for a sub-grid treatment of the IGM in reionization simulations with volumes up to 1 Gpc^3 with 7 orders of magnitude of dynamic range in the IGM opacity.

Speaker: Christopher Cain (Arizona State University)

cain_nordita_talk.pdf

Slides

16:00 Illuminating the Young Universe with FIRE: What drives the evolution of the cosmic star formation rate density?

Observations with the James Webb Space Telescope (JWST) have revealed a higher than expected ultraviolet (UV) luminosity density as well as the presence of luminous, massive galaxies already a few hundred million years after the Big Bang. The physical origin of these observational findings is not well understood but possibly related to an increased star formation efficiency at early cosmic times. In this talk, I will introduce FIREbox-HR, a new cosmological hydrodynamical simulation from the Feedback in Realistic Environments (FIRE) project, that offers insights into the star formation efficiency of galaxies during the first billion years of cosmic time. FIREbox-HR resimulates the cosmic volume of the original FIREbox run with eight times higher mass resolution down to 𝑧 ~ 6 enabling it to robustly model the abundance of galaxies across a broad range in luminosity (down to M_UV ~ -12), stellar mass, and halo mass during the Epoch of Reionization and late Cosmic Dawn (z ~ 6-15). In contrast to previous theoretical works, the UV luminosity functions and the cosmic UV luminosity density predicted by FIREbox-HR are in excellent agreement with observations out to the highest observed redshifts. Subsequently, I will discuss the implications of this finding for the star formation efficiencies of galaxies during the Epoch of Re-ionization and inferences that can be drawn for even earlier cosmic times. Future observations of UV faint galaxies at z > 12 will provide an opportunity to further test these predictions and deepen our understanding of star formation during Cosmic Dawn.

Speaker: Robert Feldmann (University of Zurich)

16:15 Stochasticity in the galaxy properties and its effect on the Epoch of Reionization

We know very little about the first galaxies that started the Cosmic Dawn (CD) and led to the Epoch of Reionization (EoR). Theory predicts these objects are highly biased, experiencing episodes of stochastic star-formation and feedback. As a consequence, radiation fields driving the CD and the EoR are expected to be very inhomogeneous. The inhomogeneity and timing of these cosmic milestones can be measured with CMB and other EoR and CD observations. We use efficient semi-numerical simulations to compute 3D lightcones of patchy EoR and other cosmic fields. We forward model the latest CMB and galaxy observations, including the patchy kinetic Sunyaev-Zeldovich signal, the Thomson scattering CMB optical depth and galaxy UV luminosity functions. We perform Bayesian inference, constraining the timing and duration of the EoR to be very late, and fairly rapid. We also find that the characteristic escape fraction of ionizing photons is $f_{\rm esc} = 4\%_{-3}^{+5}$. Additionally, we quantify the impact of different sources of stochasticity in determining galaxy emissivities during these epochs. As expected, stochasticity becomes increasingly important at earlier times when sources are rarer. We find that the star forming main sequence and the ionizing escape fraction are the most important sources of scatter in high redshift emissivities. Models seeking to accurately capture the morphology of the CD and EoR need to take this scatter into account. To constrain these models, we need to directly probe the morphology of the EoR. In order to do that we developed a new framework to infer the properties of the local HII region around a group of observed galaxies. We forward-model Lyman-alpha spectra as would be observed by JWST, dealing with all relevant sources of uncertainty. We find that the observed galaxy number densities of $n_{\rm gal} \sim 0.0025-0.00375$ cMpc$^{-3}$ should suffice to estimate the size and location of the local HII region. Such number densities are well within reach of future JWST surveys.

Speaker: Ivan Nikolić (Scuola Normale Superiore Pisa)

Nikolic_Ivan_stochasticity.pdf

16:30 Radiation-hydrodynamical modeling of the galaxy-IGM interplay during reionization

Within a decade, an array of observational facilities will deliver the first comprehensive picture of the reionizing Universe, from galaxies to the diffuse IGM. This will enable the detailed investigation of the intricate connection between the first collapsed structures and cosmic reionization. I will present my ongoing efforts to build a solid theoretical framework of study this phenomenon. In particular, I will present new results stemming from large-volume radiation-hydrodynamical simulations on the distribution of bubble sizes in the early Universe, their impact on the nearby Lyman-alpha forest and the role of galaxy properties in shaping these effects. I will show how JWST is starting to deliver important constraints on the galaxy-IGM connection during reionization, and how they can be used to constrain the properties of galaxies in the z>6 Universe. Finally, I will briefly introduce a suite of high-resolution radiation-hydrodynamical simulations that for the first time capture simultaneously the complex physics of the interstellar medium within the first galaxies and the O(100 Mpc)-scale radiation field. Through a tiered approach, these new simulations will enable to simultaneously study the first galaxies and the coeval IGM over an unprecedented range of scales.

Speaker: Enrico Garaldi (Institute for Fundamental Physics of the Universe)

16:45 Discussion

17:30 → 19:00 Welcome reception with drinks and snacks

Tuesday, 25 June

09:00 → 09:15 Information from the LOC

09:15 → 12:15 Session 3

09:15 Lyman alpha Spectroscopy with JWST: New Insights into Reionization

Speaker: Dan Stark

09:45 The impact of an evolving stellar initial mass function on early galaxy evolution and reionisation

Observations with the James Webb Space Telescope have revealed a high abundance of bright z>10 galaxy candidates that challenge the predictions of standard theoretical models, particularly at the highest redshifts. Various explanations for this discrepancy have been proposed, such as feedback-free starbursts, radiation-driven outflows clearing the dust from star-forming regions, a top-heavy stellar initial mass function (IMF), or only detecting galaxies going through periods of intense star formation due to their faster growth in mass.

We have investigated whether an IMF that becomes increasingly top-heavy towards higher redshifts and lower gas metallicities or higher gas densities could explain these observations. We have built the first model that follows early galaxy evolution and reionisation while accounting for such evolving IMFs. Specifically, we have adjusted the descriptions for supernovae feedback, metal enrichment, and ionising and ultraviolet radiation emissions in the Astraeus framework that couples an N-body simulation with a semi-analytical model for galaxy evolution and a semi-numerical model for reionisation.

In this talk, I will show how our parameterisation of evolving IMFs changes the ultraviolet luminosity functions, the relations between different galactic properties (stellar mass, star formation rate, gas-phase metallicity, dust, ultraviolet luminosity) and the topology of the ionised regions growing around the galaxies in the intergalactic medium during reionisation compared to a constant Salpeter IMF.

Speaker: Anne Hutter (Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen)

10:00 Nitrogen-Rich, Dense, and Highly-Ionized Gas in z>6 Galaxies

The discovery of an unexpectedly high N/O ratio in GN-z11 raises questions about star formation in the early universe. Using public NIRSpec data, we have pinpointed additional two galaxies at z=6.23-8.68 with nitrogen-rich gas relative to carbon and oxygen ([N/C] $\gtrsim$ 1 and [N/O] $\gtrsim$ 0.5), suggestive of predominant materials processed by the CNO cycle. Interestingly, using the NIRSpec data and our derived line-spread functions, we have found that CEERS_01019 with [OII] has a high electron density of $n_{\mathrm{e}}\sim1000$ cm$^{-3}$. Such high N/O and high $n_{\mathrm{e}}$ gas is also reported in other z>6 galaxies of GN-z11 and RXCJ2248-ID, but not in a z=6.07 galaxy of the Cosmic Grapes, suggesting a potential correlation between N/O and n_e values. Such high N/O ratios are also observed in the atmospheres of dwarf stars in globular clusters, and high $n_{\mathrm{e}}$ values of $\gtrsim1000$ cm$^{-3}$ are simulated in young massive clusters, promising GC progenitors. This suggests that GCs, which formed from dense gas with high star-formation efficiencies, may preserve nitrogen-rich gas in their dwarf stars. We also find that the 3 nitrogen-rich galaxies, CEERS_01019, GN-z11, and RXCJ2248-ID, have high NIV]/NIII] ratios beyond stellar photoionization models at a given line ratio (e.g., [OIII]/[OII]), suggestive of hard emission from black holes and/or an exotic stellar population such as Wolf-Rayet stars or supermassive stars. Notably, these sources can contribute to nitrogen enrichment by releasing the CNO-cycle materials processed in stellar outer layers via tidal disruption events or stellar winds. This talk will also delve into causal relationships among the rich nitrogen, the high $n_{\mathrm{e}}$, and the hard emission in the early universe.

Speaker: Yuki Isobe (The University of Tokyo)

10:15 Puzzlingly High N/O and N/C Galaxies at z ~ 6-10: Any Mechanisms for Insufficient C and O Enrichments

The observations conducted with JWST have disclosed galaxies at high redshifts characterized by extremely high N/O and low C/N ratios, exemplified by GN-z11. The high N/O and low C/N ratios of these galaxies are largely biased toward the equilibrium of the CNO cycle, suggesting that these three galaxies are enriched by metals processed by the CNO cycle. The low C/N and high N/O ratios of these three galaxies cannot be explained by typical chemical evolution models including asymptotic giant branch stars in their early chemical enrichment stages. We thus investigate three scenarios associated with dominant CNO-cycle materials, i.e., Wolf-Rayet stars, supermassive stars (with $10^3 - 10^5~M_\odot$ ), and tidal disruption events. We develop the chemical evolution models of these scenarios, assuming star formation based on various initial mass functions with core-collapse supernovae (CCSN) based on the theoretical yields. We find that the C/O and N/O ratios of these three galaxies are explained by any of these three scenarios. However, the N/O values of the three models decrease quickly after the CCSNe take place due to a large amount of oxygen ejected from CCSNe. Because CCSNe appear in a very short time scale (<1 Myr) for massive star progenitors, the massive stars need to directly collapse into black holes without CCSNe to prevent oxygen enrichment. This mechanism may be related to the seed black hole formation of the supermassive black holes found at high redshift.

Speaker: Kuria Watanabe (National Astronomical Observatory of Japan)

Cosmic_Dawn_202406_KuriaWatanabe.pdf

10:30 JWST bright galaxies at z > 12: a signature from Pop. III stars?

We explore the nature of the first galaxies using a semi-analytic model tailored for the rapidly evolving early Universe (Ventura et al. 2024). Applying this model to an N-body simulation enables us to resolve molecularly cooled minihalos down to ~1e5Msol within a cosmological volume of 10/h cMpc. By tracking chemical enrichment both inside and outside these first galaxies, we follow closely their stellar components and investigate the impact of Population III (Pop. III) stars on the early intergalactic medium (IGM). With this model we explored the sensitivity of the star formation rate density (SFRD) and stellar mass function (SMF) on the unknown free parameters.
In this presentation, I will demonstrate that the majority of the first galaxies are likely enriched by their own star formation with the metallicity of the IGM at z>=6 remaining below the critical value necessary to quench Pop. III star formation. However, as reionization progresses, the effects of photo-heating and photo-dissociating feedback from the UV background become more pronounced. This leads to a halt in star formation and subsequent metal production within these galaxies, with external chemical enrichment becoming more relevant (at z = 6 about 1% of the IGM volume is filled with metals).
By integrating this novel semi-analytic model with Pop. III stellar population synthesis, we predict the UV spectra and light-curve of the first galaxies, exploring how the abundance and properties of Pop III stars influence their detectability. I will discuss the intriguing prospect that the bright galaxies recently observed by JWST at z > 12 could be predominantly composed of Pop. III stars and characterized by a top-heavy initial mass function without requiring an increased star formation efficiency.

Speaker: Emanuele Maria Ventura (University of Melbourne)

CosmicDawn_Subm.pdf

10:45 Morning coffee/tea break

11:15 Formation of massive star cluster under radiative and stellar wind feedback: origins of extremely high N/O ratios and multiple stellar populations

By performing 3D radiation hydrodynamics simulations with radiative and stellar wind feedback, we study the formation of young massive star clusters (YMCs). We include the metal yield from stellar wind and supernovae. We find that the young massive star clusters are only born in the high-surface density clouds where radiative feedback becomes ineffective due to strong gravitational force from star clusters. In such a case, metal-enriched and high-density gas is formed around the star cluster. Before the supernovae occur, the high N/O ratios of gas are achieved due to stellar wind from Wolf-Rayet stars. The N/O ratios are comparable to that of discovered high-z galaxies (e.g., GN-z11). Also, some stars become nitrogen-enriched with N/O ratios similar to those of second-population stars in globular clusters (GCs). We show that metal-enriched stars are only born in star clusters more massive than 1.e6 Msun, estimating the conditions of YMC formation and metal yield due to stellar wind. These conditions of stellar multiple populations in YMCs allow us to constrain the formation site of GCs.

Speaker: Hajime Fukushima (University of Tsukuba)

CosmicDawn2024_fukushima.pdf

11:30 Inside-out growth in the early Universe

How do the morphologies of the most distant high-z galaxies evolve from cosmic dawn to the present-day? How do they build up mass and structure and what can this tell us about bulge growth, structure formation, and Early-type galaxy formation? These questions are of vital importance to both observations and simulations and are crucial in understanding the physical properties of high-z galaxies.
I will address these questions using the JADES survey, the most extensive extragalactic survey with JWST. I will present spatially resolved photometric results for a spectroscopically confirmed, redshift 7.4 galaxy with a compact central core (proto-bulge) and a strongly star-forming disc. I will explore the stellar population properties of these components and show that the radial profile of the specific star formation rate of this galaxy is sharply rising (approximately 1dex within the central 1kpc) implying rapid inside-out growth within the first 700Myrs of the universe’s existence. I will also show how, despite being 1000 times less massive than present day elliptical galaxies, the central stellar mass surface density is within 0.2-0.3dex of present day massive ellipticals, and that the expected size evolution of the galaxy would see it, by redshift 2, reaching the quiescent (early-type) population, suggesting it likely to be a progenitor to these well-studied massive quiescent (early-type) ellipticals.

Speaker: William Baker (University of Cambridge)

11:45 Probing bursty star formation in the first galaxies with JWST

JWST has opened a new chapter in our understanding of early star formation, unveiling a surprising number of luminous galaxies at z>10, as well as the first quenched low-mass galaxies. This coexistence of extreme sources, exhibiting a wide range of star formation rates, suggests a highly stochastic evolution and raises fundamental questions. Are galaxies more bursty at high-z and why? How does this affect our view of the high-z Universe?
Using a simple analytical approach, we show that due to bursty time-variable star formation histories (SFH), dark matter haloes of mass Mh can host galaxies with a broad range of luminosities. Specifically, introducing a scatter in the UV magnitudes that increases with decreasing Mh can naturally explain the observed luminosity functions up to z~12.
I will show how bursty SFHs are expected to affect the distributions of other key observables, such as UV slopes, emission lines and neutral hydrogen fractions. These predictions will help us constrain the stochasticity in high-z galaxies and unveil the physical processes regulating their evolution.

Speaker: Viola Gelli (Cosmic Dawn Center)

12:00 SPICE: Tracing the imprints of stellar feedback on reionisation era galaxies

Feedback from supernovae and radiation emitted by stars plays a pivotal role in shaping the early universe. These feedback processes have a direct influence on gas and stellar dynamics, leaving discernible traces in observational data. I introduce SPICE, a novel suite of radiation-hydrodynamical simulations targeting cosmic reionization. SPICE uses RAMSES-RT to track the propagation of radiation from stars and employs a state-of-the-art galaxy formation model with a focus on resolving the multiphase interstellar medium down to 30 pc scales. The goal of these simulations is to systematically probe a variety of stellar feedback models, including "bursty" and "smooth" modes of supernova energy injections. SPICE shows that subtle difference in the behavior of supernova feedback can drive profound difference in reionisation histories with burstier forms of feedback causing earlier reionisation. SPICE highlights that stellar feedback and its strength determine the morphological mix of galaxies emerging by z=5. While star-forming disks are prevalant if supernova feedback is smooth, bursty feedback generates dispersion dominated systems. I present a strong correlation between galaxy morphology and lyman continuum escape fractions of galaxies where dispersion supported galaxies show 20-50 times higher escape fractions as compared to their rotation dominated counterparts. Finally, I validate the observational signatures of different feedback models, as demonstrated by SPICE, against the latest data from JWST and ALMA observations.

Speaker: Aniket Bhagwat (Max Planck Institute for Astrophysics)

12:15 → 13:30 Buffet lunch

13:30 → 17:30 Session 4

13:30 The 21cm line from the high-z IGM

Speaker: Benedetta Ciardi

14:00 Towards precision constraints on reionization history with quasar IGM damping wings

Constraining the Epoch of Reionization remains one of the pivotal tasks of modern cosmology, and next-generation telescopes are opening up the path to the first precision constraints on the timing of reionization derived from the Ly-alpha damping wing signature imprinted on the spectra of high-redshift quasars by the foreground neutral intergalactic medium (IGM). In the coming years, EUCLID will detect a number of high-redshift quasars never seen before, whose exquisite spectra collected by JWST are calling for powerful statistical methods to infer precision constraints on the IGM neutral hydrogen fraction as a function of redshift.

We developed a state-of-the-art Bayesian inference pipeline that allows us to disentangle the IGM damping wing from a quasar's unknown intrinsic spectrum and infer its lifetime as well as the neutral hydrogen (HI) column density in front it, directly translating into a constraint on the global IGM neutral fraction. We account for covariances across the full spectral range caused by IGM transmission fluctuations, quasar continuum reconstruction, and spectral noise. We discuss how simulation-based inference can be leveraged to overcome non-Gaussianities in the IGM transmission likelihood by training a normalizing flow as neural likelihood estimator. These developments are facilitated by our JAX-based inference pipeline, exploiting the latest machine learning (ML) infrastructure.

After marginalizing out nuisance parameters associated with the quasar continuum, we find that we can constrain the HI column density within the first 15 pMpc of each individual quasar to 0.1 - 0.9 dex and its lifetime to 0.2 - 1.0 dex. By applying our procedure to a set of mock observational spectra resembling the distribution of EUCLID quasars with realistic spectral noise, we show that our method applied to upcoming observational data can robustly constrain the evolution of the IGM neutral fraction at the < 5% level at all redshifts between 6 < z < 11.

Speaker: Timo Kist (Leiden Observatory)

14:15 Damping wings in the Lyman-alpha forest, a model-independent measurement of the neutral fraction at 5.4<z<6.1

Recent observations have positioned the endpoint of the EoR at redshift $z \sim 5.3$. However, it has not been possible to discern whether this progression occurred slowly and late, with substantial neutral hydrogen (HI) clouds persisting at redshift $\sim 6$, or rapidly and earlier, driven by the fluctuating UV background, through observations of the Lyman-$\alpha$ forest.

Large-scale Gunn-Peterson (GP) absorption troughs (arising from absorption of UV radiation by HI in the intergalactic medium) are one of the most solid indicators that reionisation is not complete until redshift $z=5.3$, but whether they contain significantly neutral gas has not yet been proven.

We aim to solve this question by directly measuring, for the first time, the neutral hydrogen fraction ($x_\mathrm{H{\small{I}}}$) at the end of the EoR ($5 \lesssim z \lesssim 6$) in high-redshift quasars spectra.

For high neutral fractions ($x_\mathrm{HI}\gtrsim0.1$), GP troughs exhibit damping wing (DW) absorption extending over $1000$ km s$^{-1}$ beyond the troughs. While conclusively detected in Lyman-$\alpha$ emission lines of quasars at $z\geq7$, DWs are challenging to observe in the general Lyman-$\alpha$ forest due to absorption complexities and small-scale stochastic transmission features.

We report the first successful identification of the DW signal adjacent to GP troughs at redshifts $z=5.6$ and $z=6$ through careful stacking of the Lyman-$\alpha$ forest. We present a detection and measurement of the corresponding global $x_\mathrm{HI}$, made possible by the high data quality and quantity in the XQR-30 survey.
We conclusively demonstrate the existence of substantial neutral islands near the conclusion of the EoR, unequivocally signaling a late-and-slow reionization scenario.

Speaker: Dr Benedetta Spina (Heidelberg University)

14:30 Latest 21cm results from the Murchison Widefield Array

I will present updates from the past year from the Australian team in the Murchison Widefield Array (MWA) EoR experiment. With over 10 years of data acquired across three different array increments, we have more than a thousand hours of data to analyse. I will discuss the efficient Nextflow pipeline and Quality Assurance metrics developed by the EoR group, and present the latest results from the project. I will also present work by the group to model systematics in the data with a realistic end-to-end simulation pipeline, to help guide to our future methodology development.

Speaker: Cathryn Trott (ICRAR - Curtin University)

14:45 New Upper Limits of the LOFAR EoR 21 cm Power Spectrum and Their Interpretation

In this talk, I will present the current status of the field and the upper limits results, especially, I will focus on the results from the LOFAR EoR project. I will also discuss the analysis of these data in terms of cosmological models.

Speaker: Saleem Zaroubi (Kapteyn Astronomical Institute)

15:00 HERA Phase I and Beyond: Constraints on X-ray heating of the IGM

I will discuss the most recent upper limit on the 21cm Power Spectrum from the HERA Collaboration, which used 94 nights of observing with Phase I of the instrument. I will show that these most recent limits are mostly consistent with thermal noise, and represent the deepest limits to date at redshifts 7.9 and 10.4 respectively. I will also touch on the state-of-the-art validation pipeline developed by our team, and focus on the astrophysical interpretation afforded by these measurements. In particular, we rule out a broader set of "cold reionization" scenarios than has been previously possible, with ramifications for the sources of early heating. Finally, I will briefly discuss progress on upcoming data releases from HERA, which push the observing window out to Cosmic Dawn (z>13).

Speaker: Steven Murray (Scuola Normale Superiore)

Stockholm2024-HERAUpdatesH6C.pptx

15:15 Afternoon coffee / tea break

15:45 HERA bound on x-ray luminosity when accounting for population III stars

Recently, the Hydrogen Epoch of Reionization Array (HERA) has placed upper bounds on the cosmological 21-cm power spectrum at redshifts $\approx8$ and $10$. These bounds have been instrumental in constraining $L_{\rm X<2 \, keV}/{\rm SFR}$, the soft-band X-ray luminosity per unit star formation rate (SFR). Results suggest that values below $\approx 10^{39.5}\, {\rm erg} \;{\rm s}^{-1} \;{\rm M}_{\odot}^{-1} \;{\rm yr}$ are strongly disfavored. This assessment stems from analyzing semi-numerical models of the 21-cm signal which considers contributions only from atomic cooling galaxies hosting PopII stars. In this study, we reproduce these bounds on $L_{\rm X<2 \, keV}/{\rm SFR}$ and other relevant parameters by incorporating molecular cooling galaxies hosting PopIII stars into the simulation of the cosmic dawn 21-cm signal. We demonstrate that with the inclusion of molecular cooling galaxies, the lower values of $L_{\rm X<2 \, keV}/{\rm SFR}$ are no longer strongly disfavored. Consequently, this suggests that high-redshift X-ray sources may not necessarily need to exhibit significantly higher X-ray luminosity than high-mass X-ray binaries observed at lower redshifts.

Speaker: Debanjan Sarkar (Trottier Space Institute and Department of Physics, McGill University)

16:00 Capturing the HI 21 cm Signal in Ground-Based Observations Amidst Diverse Systematic Influences

The Epoch of Reionization (EoR) and Cosmic Dawn (CD) play crucial roles in shaping the early Universe during its initial billion years. Despite their significance, the characteristics of the Intergalactic Medium (IGM) during these epochs remain uncertain and require further observational validation. Current and upcoming radio telescopes, such as EDGES, SARAS, MWA, and SKA, aim to detect the redshifted HI 21-cm signal, a key objective in understanding these cosmic periods. However, challenges arise from systematic errors and the need for precise foreground removal.
Especially at lower frequencies, the Earth's ionosphere introduces significant distortions, creating direction-dependent effects on the signal. Therefore, it is imperative to comprehend the impact of each source of corruption when employing non-parametric techniques for accurate 21cm signal detection. This presentation is divided into two parts:
i) We will introduce an end-to-end pipeline developed to investigate the influence of bright foreground, primary beam, ionosphere, and other systematics on extracting the redshifted HI 21cm signal from upcoming interferometric experiments like SKA-1 Low. This pipeline incorporates various signal extraction methods, with a specific focus on power-spectrum estimation. Simulation is employed to understand and quantify how different ionospheric scale structures affect the precision of 21-cm signal power spectrum estimation in synthetic observations with SKA1-Low.
ii) We will discuss parameter estimation using Artificial Neural Networks (ANN) and Markov Chain Monte Carlo (MCMC) to extract the HI 21-cm power spectrum and associated parameters from observed sky signals using this end to end pipeline. This involves considering the HI signal, actual foregrounds, calibration and position errors, as well as systematic effects.

The developed end-to-end pipeline enhances the accuracy of ground-based data analysis for upcoming radio interferometric studies like SKA, MWA, and HERA. This improvement contributes to a better understanding of ionospheric effects and systematic error tolerance levels across different arrays and their impact on parameter estimation.

Speaker: Abhirup Datta (DAASE, IIT Indore)

16:15 Study of the diffuse Galactic synchrotron emission using MWA Drift Scan Observations

The redshifted 21 cm line is an emerging tool in cosmology that helps us understand the large-scale structure of the universe. Drift-scan observations, where the direction in which the telescope's point changes continuously on the sky due to the earth's rotation, provide an economical and stable option if one desires broad sky coverage. However, the Galactic and extra-galactic foreground become a severe challenge for the detection of the 21-cm signal. In this talk, I will present a novel estimator, namely the Tapered Gridded Estimator for measuring the 21-cm power spectrum from radio interferometric visibilities. Also, I will present our recent results using MWA phase II observations to characterize the diffuse Galactic foreground emission. This study will help to model the foreground properly, and also remove them accurately toward detecting the cosmological 21-cm signal.

Speaker: Samir Choudhuri (Indian Institute of Technology Madras)

16:30 Constraining the clustering and 21-cm signature of radio galaxies at cosmic dawn

Several ongoing and upcoming radio telescopes are aiming for a detection of either the all sky-averaged (global) 21-cm signal or the 21-cm power spectrum. The extragalactic radio background, as detected by ARCADE-2 and LWA-1, can significantly affect the cosmological 21-cm signal. Such a radio background could have been produced by high-redshift galaxies. We perform a complete calculation that accounts for the fact that the 21-cm absorption occurs along the line of sight, and is therefore sensitive to radio sources lying behind each absorbing cloud. We find that the complete calculation strongly enhances the 21-cm power spectrum during cosmic dawn. Furthermore, using this model, we constrain the clustering of high-redshift radio sources based on observed upper limits from the VLA and ATCA. The results show significantly stronger clustering constraints on radio efficiency compared to those from overall background intensity. Consequently, the predicted maximum depth of the global 21-cm signal and the maximum 21-cm power spectrum peak at cosmic dawn are both lowered. In conclusion, observed clustering stands as the strongest current direct constraint on such models.

Speaker: Sudipta Sikder (Tel Aviv University)

16:45 Discussion

Speakers: Benedetta Ciardi, Dan Stark

Wednesday, 26 June

09:00 → 09:15 Information from the LOC

09:15 → 12:30 Session 5

09:15 How useful are simulations for understanding reionization and its sources?

Speaker: Joakim Rosdahl

09:45 Galaxies as agents of cosmic reionization: new results from JWST WFSS and VLT/MUSE observations

Reionization marks the last major phase transition of matter in the Universe and its completion had crucial impact on the formation of the smallest galaxies. While reionization roughly encapsulated the first Gyr of cosmic time, the precise timing, topology and the sources of ionizing photons are unknown. In my talk, I will mainly present results on the spectroscopic properties of galaxies in the early Universe from the EIGER and ALT surveys, which total ~180 hrs of JWST/NIRCam imaging and wide-field slitless spectroscopy in the fields of high-redshift luminous quasars and the Abell 2744 lensing cluster. I will show how we used these data to confirm redshifts for ~2000 galaxies at z=3-9, and simultaneously measured their distributions of star formation rate, mass, metallicity, UV colours and line equivalent widths. I will also present our results on the abundance and properties of faint AGN at high redshift, and discuss the current insights on their importance for understanding supermassive black hole formation and their role in cosmic reionization. Finally, I will highlight avenues to address current fundamental limitations in our understanding of the role of galaxies in the reionization of the Universe with new and future VLT programs focussed on the Lyman-alpha emission-line, including the first galaxy tomographic study at high-redshift in the MUSE eXtremely Deep Field.

Speaker: Jorryt Matthee (Institute of Science and Technology Austria)

10:00 Radiation-Driven Superwinds and Lyman Continuum Escape in Local Reionization-Era Analogs

A key unknown in our understanding of cosmic reionization is the feedback mechanisms that enabled Lyman continuum (LyC) to escape in the earliest starbursts. One clue comes from Green Pea (GP) galaxies, the largest class of local (z ~ 0.3) LyC emitters. Most GPs show broad nebular emission-line wings of unknown origin, reaching 200-1000 km/s. The broad-wing velocity has been shown to correlate with the LyC escape fraction, indicating a link between LyC escape and feedback. We now investigate whether the broad wings of Low-redshift Lyman Continuum Survey (LzLCS) galaxies trace radiation-driven superwinds, as proposed by Komarova et al. 2021 (K21). Using Magellan/MIKE, WHT/ISIS, and VLT/X-Shooter spectra of 28 LzLCS galaxies, we relate the broad-wing parameters to galaxy and LyC properties. We find that broader, brighter wings are correlated with high star formation rate density, high $O_{32}$, young age, and low metallicity, consistent with radiation-driven winds. The broad-wing velocity and luminosity moreover correlate with the leaking LyC luminosity, in agreement with superwinds accelerated by LyC/Ly$\alpha$ photons. Indeed, based on the K21 model, we show that most of the observed broad-wing velocities can only be explained with H I opacity, or LyC/Ly$\alpha$-driving. So, radiation-accelerated, and in particular LyC/Ly$\alpha$-driven, superwinds may be an important feature tracing LyC escape. Given the abundance of metal-poor, high-ionization starbursts in the early universe revealed by JWST, such superwinds are likely to be common in the reionization era.

Speaker: Lena Komarova (University of Michigan)

Lena Komarova.pdf

10:15 Are local diagnostics for LyC escape fraction valid during the EoR?

The escape fraction of Lyman Continuum radiation (f_esc) remains the most uncertain parameter in modeling the epoch of reionization. Depending on its value, and its dependency on galaxy luminosities, different ionization histories and topologies are expected. Photons more energetic than 13.6eV escaping from a galaxy are readily absorbed in the intergalactic medium (IGM) making the f_esc measurement practically impossible at z<3.5.
I will present the results of a new HST study constraining the escape of ionizing radiation in a sample of seven gravitationally lensed galaxies at z~2.4. The lensing magnification allows us to push the measurement of f_esc to faint magnitudes. The galaxies all have Lyman alpha in emission and low dust content (inferred from the slope of the UV continuum), suggestive of high f_esc. We find, however, that our targets are not detected in Lyman continuum, with absolute escape fraction upper limits of ~2%. Our findings disagree with results from previous studies based on local galaxy analogs, suggesting that blindly applying z=0 diagnostics to reionization epoch galaxies may be incorrect.

Speaker: Claudia Scarlata

10:30 LyC Escape and IGM Tomography Using the 600-900Å Continuum of the Sunburst Arc

The Sunburst Arc (z~2.37) is a confirmed Lyman continuum leaker galaxy and has been uniquely lensed where its single leaking region is imaged 12 times over four separate arcs. Using HST/WFC3 UVIS G280 grism observations, we extracted the spectra of the leaking region to determine the shape of the stellar continuum from 600-900Å (produced by young, massive O and B stars) for the first time. The intrinsic Lyman continuum escape fraction is >70% leaving the interstellar medium, but foreground intergalactic medium (IGM) absorption lowers this estimate to ~30%. The same foreground IGM absorbers at z~2 are seen in multiple images, allowing us to constrain their physical size in 2D and estimate their HI mass. This rare sightline gives insight into the conditions necessary for the propagation of ionizing photons and can help interpret results from reionization.

Speaker: Michelle Berg (University of Texas at Austin)

10:45 Morning coffee/tea break

11:15 The impact of mergers on the Lyman Continuum photons escape in Galaxies during the epoch of reionization

The opacity of the intergalactic medium does not allow a direct measurement of Lyman continuum (LyC) emission at redshifts exceeding 4.5. Consequently, we are increasingly relying on indirect indicators of LyC emission that are well tested at low and intermediate redshift. In this study, we use several such indirect indicators, based on parameters like UV-beta slope, E(B-V), UV magnitude, and EW([O III]+Hβ) to refine estimation of LyC emission for galaxies in the epoch of reionization. Leveraging on a very large statistical sample comprising over 750 confirmed sources in the epoch of reionization from several spectroscopic JWST surveys, we analyze the statistical contribution of galaxies to cosmic reionization. In addition, combining NIRCAM imaging and spectroscopy we investigate the relationship between predicted fesc fractions and the morphological properties of the sources (such as clumpiness and asymmetry). Our aim is to unveil the mechanisms that favor the escape of Lyman continuum photons, and understand the complex interplay between galaxy structure and Lyman continuum radiation. In particular we investigate if mergers and interactions can play a significant role in the formation of channels through which the LyC radiation can escape, for example by offsetting the neutral gas from the center of the galaxies where the LyC emission production is located as recently found in the local leaker Haro11.

Speaker: Sara Mascia (INAF OAR, Università degli Studi di Roma "Tor Vergata")

Mascia.pdf

11:30 Influence of galactic mergers on the escape of LyC radiation during reionization

Low mass galaxies are one of the main sources of Lyman continuum (LyC) photons, which ionized the intergalactic medium during cosmic reionization. However, the escape fraction of LyC photons from these galaxies depends on the properties and dynamics of the neutral gas that surrounds the star-forming regions. Galactic mergers can affect both the star formation rate and the gas distribution allowing LyC photons to leak out. In addition, tidal forces can compress gas within the galaxy resulting in additional star-formation.

In this talk, I will present the results of a study that investigates how LyC escape changes after a galactic merger, using post-processed galaxies from the TNG50 cosmological simulation. I will show how the merger history, the star formation rate, and the gas morphology influence the LyC escape fraction and discuss the implications for cosmic reionization.

Speaker: Ivan Kostyuk (Max Planck Institute for Astrophysics)

11:45 Fluctuations and Evolution of the Ionizing Background and Mean Free Path during the Late Stages of Reionization

Measurements of the ionizing background and the mean free path (MFP) at the end of reionization offer valuable insights into the first generation of sources that ionized the universe. With increasing observational data from this epoch, a better theoretical understanding is imperative to interpret the data accurately.
The CROC simulations are uniquely poised to interpret observational data, with both high spatial resolution to resolve galaxy formation and fully-coupled radiative transfer to authentically simulate the reionization of the intergalactic medium (IGM). We analyze two CROC boxes with distinct reionization histories to gain insights into the uniformity of the ionizing background and MFP of photons. We find that in the late reionization box, where the volume-weighted neutral fraction x_HI drops to 0.5 at z=7.4 and below 0.001 at z=6.4, the ionizing background still displays significant fluctuations (~40%) at z=5. These fluctuations are closely related to the non-uniform distribution of ionizing sources and Lyman Limit Systems (LLSs). Additionally, we investigate the mean free path (MFP) and observe differences across different environments. Notably, the MFP around very massive halos (>1e12 Msun) is significantly smaller than in average regions of the universe, attributed to the abundance of LLSs.
Observations measure the mean free path (MFP) using quasar spectra, where the ionization of Lyman Limit Systems (LLSs) around quasar hosts must be considered. For consistent comparison to observations, we create thousands of mock quasar spectra by post-processing the sightlines centered on massive halos, and assess the accuracy of the observational procedure outlined in Becker+21. Our analysis quantifies the bias in the inferred MFP, highlighting important implications for interpreting observational data and understanding the end stage of reionization.

Speaker: Huanqing Chen (Canadian Institute for Theoretical Astrophysics)

12:00 The forest at EndEoR: The effect of Lyman Limit Systems on the End of Reionisation

The Epoch of Reionisation (EoR) was the period of the Universe during which the neutral hydrogen present in the intergalactic medium (IGM) was ionised by Ultraviolet radiation from sources inside early galaxies. Significant progress has been made in modelling cosmic reionisation through numeric and semi-numeric codes. However, the end of reionisation has been studied less. In radiative transfer simulations, the opacity of the ionised IGM for ionising photons becomes important during the EoR's end, and there is quite a bit of uncertainty about this quantity. While Ultraviolet Background (UVB) measurements at z = 6 exist, many simulations struggle to reproduce this number and report higher UVB values.

In this talk, I address how the final stages of reionisation depend on assumptions about the sources, the clumping of the intergalactic medium and the presence of small-scale absorbers (so-called Lyman Limit Systems). In large-scale simulations of the EoR, the latter are often modelled through a parameter to account for the mean free path of ionising photons (MFP) in the IGM. We find that a shorter MFP, more consistent with current measurements from Becker et al. 2021, not only extends reionisation but regulates the UVB and IGM opacity. Moreover, we show how the numerical implementation of MFP impacts the evolution of the UVB.

Speaker: Ivelin Georgiev (Stockholm University)

12:15 Impact of Self-shielding Minihalos on the Lya Forest at High Redshift

Dense gas in minihalos can shield themselves from reionization for ~100 Myr after being exposed to UV background. These self-shielded systems, often unresolved in cosmological simulations, can introduce strong absorption in quasar spectra. We present the first systematic study on the impact of these systems on the Lya forest. We find that the incidence rate of damped Lya absorbers increases by a factor of ~2 - 4 between z=4.5 and 5.5, and the Lya flux is, on average suppressed by ~3% of its mean due to minihalos. The absorption features enhances the 1D power spectrum by up to ~10 % at k~0.1 h/Mpc. The flux is particularly suppressed in the vicinity of large halos along the line-of-sight direction. However, these effects become much smaller for higher ionizing rates (>~ 10^-12/s) expected in the post-reionization universe. Our findings highlight the need to consider minihalo absorption when interpreting the Lya forest at z>~5.5. Moreover, the sensitivity of this effect to the ionizing background intensity can be exploited to constrain the intensity itself.

Speaker: Hyunbae Park

12:30 → 18:30 Free afternoon

19:00 → 21:30 Conference dinner at restaurant Josefina

Thursday, 27 June

09:30 → 09:45 Information from the LOC

09:45 → 12:45 Session 6

09:45 JWST Constraints on Early Galaxy Formation and AGN Activity at z=4-14

We present number densities and physical properties of galaxies spectroscopically confirmed with JWST up to z~14. The UV luminosity functions determined with the spectroscopic galaxy sample are consistent with the photometric results from previous studies, and show a tension with most of the theoretical model predictions. We will discuss several possibilities that can explain the observed overabundance of bright galaxies at z>10. We also present JWST discoveries of abundant AGN populations at z>4 and their physical properties, important to understand the nature of these newly-found objects.

Speaker: Yuichi Harikane

10:15 Anatomy of an ionized bubble: NIRcam Grism spectroscopy of z=6.6 double-peaked Lyman-alpha emitter COLA1 and its environment

There is an ongoing debate of whether bright or faint galaxies reionized the Universe. In this context, the galaxy COLA1 - a luminous rare double peaked Lyman-$\alpha$ (Ly$\alpha$) emitter in the COSMOS field at a redshift $z=6.6$ - provides an exceptional opportunity to unveil the anatomy of an ionized bubble in detail. The visibility of the blue peak of the Ly$\alpha$ line suggests that COLA1 resides inside a large highly ionized bubble, and allows us to infer the Lyman Continuum escape fraction. However, such ionized bubbles have not been found in recent reionization simulations. I will present the first view of COLA1 and its large-scale environment obtained from Wide Field Slitless Spectroscopic (WFSS) data from JWST/NIRCam. First, we prove that COLA1 is a single extremely compact bright galaxy ($M_{\rm UV}=-21.3$). We confirm the systemic redshift suggested by the Ly$\alpha$ double-peaked profile using detections of H$\gamma$, H$\beta$ and [OIII]. I will discuss how we measure an exceptionally high Ly$\alpha$ escape fraction of $f_{\rm esc}({\rm Ly}\alpha)=81\pm 5\%$ and how COLA1 has all the properties expected for strong ionizing galaxies with an inferred ionizing photon escape fraction of $f_{\rm esc}\approx 30\%$. Using the complete spectroscopic coverage of the WFSS mode, we identify a total of 141 spectroscopically selected [OIII] emitters in the $\sim 22$ arcmin$^2$ field at $z\sim 5$-$7$. While we find an overdensity ($\delta+1 = 1.96$; 4 galaxies) around COLA1, we use a semi-empirical forward modeling approach to show that this is a totally normal environment given COLA1s UV luminosity. Our results suggest that COLA1 itself makes a substantial contribution to the ionizing photon budget needed to power its bubble.

Speaker: Alberto Torralba-Torregrosa (UV)

CosmicDawn_2024_ATT (2).pdf

10:30 Stellar and AGN Feedback Probed with Outflows in JWST Galaxies at z=3-9: Implications of More Frequent and Spherical Galactic Fountains

We study outflows in 130 galaxies with 16<M_UV<-22 at z=3-9 identified in JWST spectroscopic data taken by the CEERS, ERO, FRESCO, GLASS, and JADES programs. We identify 30 out of the 130 galaxies with broad components of FWHM ~150-800 km/s in the emission lines of Hα and [OIII]5007 that trace ionized outflows. Four out of the 30 outflowing galaxies are Type 1 AGNs whose Hα emission lines include line profile components as broad as FWHM>1000 km/s. With the velocity shift and line widths of the outflow broad lines, we obtain ~80-500 km/s for the outflow velocities. We find that the outflow velocities as a function of star-formation rate are comparable to or higher than those of galaxies at z~1 while the outflow velocities of AGNs are large but not significantly different from the others. Interestingly, these outflow velocities are typically not high enough to escape from the galactic potentials, suggestive of fountain-type outflows. We estimate mass-loading factors to be η=0.1-1 that are not particularly large, but comparable with those of z~1 outflows. The large fraction of galaxies with outflows provides constraints on outflow parameters, suggesting a wide opening angle of $>45$ deg and a large duty-cycle of $>30\%$, which gives a picture of more frequent and spherical outflows in high-z galaxies.

Speaker: Yi Xu (The University of Tokyo)

Cosmic Dawn Yi Xu.pdf

10:45 The emergence of high-redshift dusty galaxies in the Obelisk simulation

The new generation of facilities such as the JWST has opened a new window on the high-redshift Universe. One of the key early results of these recent observations, confirming the findings of large ALMA surveys such as REBELS or ALPINE, is the presence of dust in galaxies already early in the history of the Universe.
In the context of the Epoch of Reionisation, this raises two questions: can we understand how galaxies assembled their dust content in a billion year or so? and how does this presence of dust affects the contribution of galaxies to the Reionisation of the Universe?
In the recent years, cosmological simulations have started to incorporate the detailed ISM models accounting for evolution of a dust component, giving a first pathway to answer these questions.

In this talk, I will present recent results obtained with Obelisk, a cosmological radiation-hydrodynamics simulation modelling the evolution of an overdensity and its large scale environment down to $z \sim 3.5$. I will describe the growth of the dust content of high-z galaxies in the simulation. I will then discuss how the resulting observable properties of the galaxy population in Obelisk is affected by the presence of dust, and how it compares to real-world observations in the context of deep JWST surveys. I will focus in particular on the role of the extinction curve. Finally, I will present a first estimate of how the dust in these galaxies interacts with the escape of ionising photons, and compare this in particular to $z\sim 0.3$ analogues taken from the Low-z Lyman Continuum Survey.

Speaker: Maxime Trebitsch (Kapteyn Astronomical Institute)

NORDITA2024_Trebitsch.pdf

11:00 The Inhomogeneous Rise of Metallicity During the Epoch of Reionization

When galaxies and stars began to form, they released ionizing radiation into the intergalactic medium which resulted in its reionization over the course of the first billion years. This ionizing radiation was dominated by massive stars. Reionization was inhomogeneous in space and time, reflecting the clustering of galaxies, and the inhomogeneous density field into which their radiation caused ionization fronts to propagate, resulting in different arrival times of those ionization fronts at different locations. The same massive stars that released this ionizing radiation also formed and released heavy elements when they exploded as supernovae, enriching the metal-free primordial gas both inside galaxies and outside them, by driving winds into the surrounding IGM. Just as reionization was inhomogeneous, so must the rise of metallicity during the Epoch of Reionization (EoR) have been. The theory of this inhomogeneous rise of metallicity is, therefore, inseparable from the theory of reionization, and predicting its observable consequences requires us to model both processes, together, self-consistently. As JWST pushes the observational frontier, to observe galaxies and detect absorption lines in the intergalactic medium during and after the epoch of reionization, we must predict and interpret what it sees. Towards this end, we have analyzed the results of the latest state-of-the-art radiation-hydro simulation of fully-coupled galaxy formation and reionization by The Cosmic Dawn (“CoDa”) Project, CoDa III, including its self-consistent tracking of the inhomogeneous rise of metallicity through the end of the EoR and beyond, down to z = 4.6. CoDa III is the first trillion-element radiation-hydro simulation of the EoR, with enough dynamic range to resolve the formation of every star-forming galaxy believed to be responsible for reionization (and, hence, metallicity) in its simulation volume of 94.4 cMpc across. We will present these CoDa III results for the inhomogeneous evolution of metallicity in the universe and its correlation with the inhomogeneity of reionization, itself, and their observable consequences.

Speaker: Joohyun Lee (UT Austin)

Joohyun_Lee_Cosmic_Dawn_at_High_Latitudes_6.27.24.pdf

11:15 Morning coffee/tea break

11:45 Unbiased UV continuum slopes at z=6.5-13 from combined PEARLS GTO and public JWST NIRCam imaging

In this talk, I will present UV continuum properties from the EPOCHS sample of 1011 high-redshift galaxies spanning z=6.5-13 across 179 square arcmin of public and PEARLS GTO NIRCam imaging from JWST Cycle 1. I will show the bias corrected UV beta-MUV relation, from which I find that the MUV=-19 galaxy population becomes extremely blue at z>11. We find that 68 of these candidates are robust ultra-blue galaxies with beta<-2.8. Performing Bayesian SED fitting on the candidates shows that, as expected, they host non-zero Lyman continuum escape fractions and hence may be important contributors to the reionization of the Universe at early times. I show that these extremely blue systems could be due to the presence of strong Lyman alpha emission (e.g. Saxena+23), which can bias beta by as much as 0.6 bluewards. This would imply large ionized bubble sizes at these epochs, which has been seen in early spectroscopic results with JWST. In addition, I find a large population of z=6.5-11 low mass, red galaxies not seen in HST studies, which could be due to damped Lyman alpha systems or the Lyman alpha damping wing, both of which bias beta red in NIRCam wideband surveys. Finally, I find that our observed average beta values at z=11-13 are consistent with the theoretical models of Pop. III stars from Zackrisson+11 mixed with more-metal enriched systems, although several degeneracies exist here highlighting the importance of NIRSpec spectroscopic data for these sources.

Speaker: Duncan Austin (University of Manchester)

12:00 The Power of Local Analogues in Understanding the ISM Conditions of High-z Galaxies

Rest-frame UV spectra play a key role in the understanding of massive stellar populations, chemical evolution, feedback processes, and reionization. In particular, in the current JWST era, the UV spectroscopic frontier has been pushed to higher redshifts than ever before, to finally reveal the first galaxies in the distant Universe. It is thus fundamental to understand the diagnostic power of UV spectral features. To this end, I will discuss how HST UV spectra of local high-z analogues from CLASSY (COS Legacy Archive Spectroscopic SurveY) can represent a powerful ideal laboratory, thanks to the level of data quality and spatial resolution the local Universe can offer. The sample is representative of SFGs across all redshifts, including extremely metal-poor objects similar to reionization-era systems. As such, we can tailor a complete UV diagnostic toolkit to explore the interstellar medium (ISM) properties (i.e., density, temperature, gas-phase metallicity, ionization parameter, source of ionization, star formation rate). In this talk I will present such a toolkit, obtained from the analysis of the main emission lines of CLASSY spectra and the comparison with well-known optical diagnostics, and discuss both benfits and challenges in using UV ISM diagnostics at high-z. Moreover, using UV ISM absorption lines, I will showcase how comparing abundances in the neutral vs ionized gas can provide us with essential insight into enrichment scenarios and mixing timescales in high-z systems. Overall, this talk demonstrates how powerful local analogues can be in understanding the ISM conditions of the earliest galaxies.

Speaker: Bethan James (STScI)

12:15 The Resolved Dust and Interstellar Medium of a z=7.31 Rotating Disk

Over the last decade, ALMA has revolutionized our understanding of the interstellar medium (ISM) conditions of distant galaxies. For one, ALMA has now detected (sub-)millimeter continuum emission from dozens of galaxies at $z > 6.5$, establishing the importance of dust-obscured star formation already within the first 800 Myr after the Big Bang. Moreover, through various bright emission line diagnostics such as [CII]$_{158}$ and [OIII]$_{88}$, ALMA can be used to directly study the physical conditions and kinematics of the ISM within the earliest galaxies. In this talk, I will first present new ALMA continuum observations of a dusty galaxy at $z=7.31$ in six unique bands (including Band 9) which meticulously constrain its dust SED and demonstrate it to host a massive dust reservoir. Producing such a large dust mass so shortly after the Big Bang likely requires efficient grain growth in a dense ISM, on top of efficient dust production from supernovae. This suggests the galaxy to be a rather evolved system already by $z > 7$, which is supported by recent high-resolution ($\sim0.2''$) [CII] observations demonstrating it to be a rotating disk — the most distant cold disk known to date. I will present matched-resolution Band 8 dust continuum and [OIII] observations of this spectacular galaxy, and discuss its multi-band kinematics, star formation and ISM properties at high spatial resolution.

Speaker: Hiddo Algera (Hiroshima University)

12:30 Constraining galaxy close pair fractions and merger rates from Cosmic Noon to Cosmic Dawn using deep JWST observations

Galaxy mergers play a crucial role in the mass build-up and evolution of galaxies according to hierarchical structure formation and cosmological simulations. I will present the most extensive study of major galaxy mergers (with a stellar mass ratio of 1:4) to date, covering the yet poorly understood and mainly unexplored redshift range of z ∼ 2−10. I use the NIRCam imaging and NIRSpec spectroscopic datasets from the JADES GTO programme, which is the most extensive JWST programme. I focus on measuring the close pair fraction using a probabilistic method incorporating the full posterior distribution and uncertainties related to the parameters obtained from photometric redshift and SED fitting. Galaxies are carefully selected only to include the highest quality photometric redshift posteriors to form a mass-complete sample and account for potentially missing objects due to selection effects and incompleteness in the form of different statistical weights. The selection criteria for finding close pairs include stellar mass ratio for major mergers, projected 2D separation proximity, and substantial overlap between the photometric redshift posterior distributions. I also correct for potential objects missing due to survey boundaries limiting the search area and assess the effects of cosmic variance. Pair fractions are then evaluated in different stellar mass bins at a wide range of cosmic epochs, with uncertainties obtained by bootstrapping analysis. The close pair fractions are turned into galaxy merger rates by assuming a characteristic merger timescale obtained from cosmological simulations. The pair fractions found show an increasing trend with redshift at lower stellar masses and a constant or slightly declining trend at the higher stellar mass bins. The resulting merger rate still agrees with simulations showing an increase as the dynamical timescale gets shorter at higher redshifts.

Speaker: David Puskas (Kavli Institute for Cosmology, University of Cambridge)

12:45 → 14:00 Buffet lunch

14:00 → 17:30 Session 7

14:00 New tools and simulations for detecting the Reionization Era Neutral Hydrogen 21-cm Signal

Within the LOFAR EoR KSP team, Gaussian Process Regression (GPR)  has been used for foreground subtraction from data, to constrain the Neutral Hydrogen 21-cm signal power spectrum from the Epoch of Reionization. To avoid signal loss due to the misestimation of the covariance kernel for the 21-cm signal, we developed a Machine Learning (ML) trained model, by training on a large variety of N-body + 1D radiative transfer simulations (GRIZZLY). We tested its limitations by exploring a variety of mock datasets. Now I will show the results of implementing it on 10 nights of observational data from the LOFAR telescope at z~9.1. 
Additionally, I will showcase a new suite of simulations (N-body with GADGET-4 + Semi-Analytic Modelling with L-Galaxies + 1D radiative transfer with GRIZZLY) exploring a range of cosmologies (varying h0 and sigma8) for a wider training set for the ML model as well as for data interpretation from tighter upper limits from future observations with LOFAR, SKA, etc. I will also show improvements using the Fixed & Paired method to reduce cosmic variance and discuss future improvements via subgrid modelling, allowing us to improve resolution. This suite of simulations also provides mock observations for telescopes like JWST and Euclid, thus providing a platform for multi-wavelength studies.

Speaker: Anshuman Acharya (Max Planck Institute for Astrophysics, Garching)

CDatHighLats_Stockholm_AnshumanA.key

14:15 A new method to simultaneously determine the reionization history and power spectrum

The light-cone (LC) effect arises from the cosmological evolution of the redshifted 21-cm signal along an observer’s line of sight (LoS), which is the frequency axis. It is particularly pronounced during the Epoch of Reionization (EoR) when the mean neutral hydrogen fraction (x_HI) and statistical properties of the universe change rapidly. The 3D power spectrum only quantifies the ergodic part of the signal and fails to capture the variation of the x_HI. The multi-frequency angular power spectrum (MAPS), on the other hand, quantifies the entire second-order statistics of the signal in the presence of the LC effect, including the homogeneous and isotropic statistical fluctuations along all three spatial directions and systematic variation along the LoS direction due to cosmological evolution. We propose a new method to observationally determine the reionization history along with the power spectrum by assuming a model where the systematic frequency dependence of MAPS arises entirely due to the evolution of the mean neutral hydrogen fraction, whereas the ergodic part of the MAPS carries the information of the homogeneous and isotropic statistical fluctuations. We validate our method by calculating MAPS from simulated LC EoR 21-cm datasets for different statistically independent realizations and using the Markov Chain Monte Carlo (MCMC) method. We demonstrate that it is possible to determine the reionization history across the entire observational bandwidth and the power spectrum monopole and quadrupole moments simultaneously using the complete information in the observed MAPS.

Speaker: Suman Pramanick (Indian Institute of Technology (IIT) Kharagpur)

14:30 Redshifted 21-cm bispectrum from CD-EoR: Impact of Source Model and IGM Physics

The radiations from the first luminous sources drive the fluctuations in the HI 21-cm signal at Cosmic Dawn (CD) via two dominant astrophysical processes i.e. the Ly$\alpha$ coupling and X-ray heating, making this signal highly non-Gaussian. The impact of these processes on the 21-cm signal and its non-Gaussianity vary depending on the properties of these first sources of light. Considering different CD scenarios by varying two major CD source parameters i.e. the minimum halo mass $M_{h,min}$ and X-ray photon production efficiency $f_X$ in a 1D radiative transfer code GRIZZLY, we study their impact on the large scale 21-cm bispectrum for all possible unique $k$ triangles in the Fourier domain. Our detailed and comparative analysis of the power spectrum and bispectrum shows that the shape, sign and magnitude of the bispectrum combinedly provide the best measure of the signal fluctuations and its non-Gaussianity compared to the power spectrum. We also conclude that it is important to study the sequence of sign changes along with the variations in the shape and magnitude of the bispectrum throughout the CD history to arrive at a robust conclusion about the dominant IGM processes at different cosmic times. We further observe that among all the possible unique k-triangles, the large-scale non-Gaussianity in signal is best probed by the small k-triangles in the squeezed limit and by triangles of similar shapes. This opens up the possibility of constraining the source parameters during the CD using the 21-cm bispectrum.

We next extend our analysis to the Epoch of Reionization (EoR), where the fluctuations in the HI 21-cm signal are mostly driven by the ionization of the HI in the IGM by these first sources of light. We study the 21-cm bispectrum for different reionization scenarios assuming different simulated models for the sources of reionization. We demonstrate how well the 21-cm bispectrum can distinguish between different EoR 21-cm signal topologies (defined by the size and distribution of ionized regions), which will help us shed light on the nature of the sources of ionizing photons. Our estimated large-scale bispectrum for all unique $k$-triangle shapes show a significant difference in their magnitude and sign, as one goes from one reionization scenario to the other. Additionally, our focused analysis of bispectrum for a few specific $k-\text{triangle}$ shapes (e.g. squeezed limit, linear and shapes in the vicinity of the squeezed limit) show that the large scale 21-cm bispectrum can distinguish between reionization scenarios that show inside-out, outside-in and a combination of inside-out and outside-in topologies. These results highlight the potential of using the 21-cm bispectrum for constraining different reionization scenarios. Finally, we establish the richer information content of the EoR 21-cm bispectrum compared to the power spectrum through a Fisher information matrix analysis.

Speaker: Suman Majumdar (Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore)

Suman_Majumdar.pdf

14:45 Prospects of a statistical detection of the 21-cm forest and its potential to constrain the cosmic heating and reionization history

The cosmological 21-cm forest, a series of absorption lines in the spectra of high-z radio-loud sources arising from the hyperfine structure of neutral hydrogen residing in the intergalactic medium (IGM), has a potential to be a unique probe of the neutral IGM during the Epoch of Reionization. I will argue that the prospects of detecting the 21-cm forest signal have improved recently because of (1) recent evidence that reionization ended as late as z<5.5 and (2) increase in the number of known high-z radio-loud quasars. In this context, I will present our models of the 21-cm forest signal based on cosmological simulations, in which we simultaneously vary the X-ray background radiation efficiency and ionization state of the IGM. I will discuss the detectability of this signal by the uGMRT and SKA1-low, both direct detection of individual absorption lines and statistical detection. I will finish this talk by showing that the spectroscopical observations of the 21-cm forest signal provide a unique opportunity to constrain the cosmic heating and reionization history at z≥6 even in the case of a null-detection.

Speaker: Tomáš Šoltinský (Tata Institute of Fundamental Research)

15:00 Understanding the epoch of reionization with MOSAIC galaxy - SKA 21cm cross correlation

The Epoch of Reionization (EoR) remains one of the last frontiers of observational cosmology. The Square Kilometer Array (SKA) is poised to make a preliminary detection of the 21-cm EoR signal within the next decade, and the race will ignite for a confirmation of this detection. One of the most promising avenues is through cross-correlation with galaxies in the same field. I present a framework for self-consistently simulating 21-cm and galaxy fields and then computing their cross-power spectrum, including a forecast of the achievable signal-to-noise ratio of a cross-correlation measurement made using the SKA and the European Large Telescope (ELT) MOSAIC instrument. We demonstrate that a MOSAIC Lyman alpha emitter survey overlapping with one square degree of the SKA field at a depth of 27.3 AB-magnitudes can achieve S/N~8 in under 100 hours of observation.

Speaker: Samuel Gagnon-Hartman (Scuola Normale Superiore di Pisa)

Stockholm Week 3.pptx

15:15 Afternoon coffee / tea break

15:45 Probing the Epoch of Reionization with line-intensity mapping

Line-intensity mapping (LIM) has emerged as a novel technique to probe the large-scale structures in the Universe. It is expected to help probe the Epoch of Reionization, the poorly understood era when the first luminous sources formed in the Universe and reionized the surrounding neutral gas in the IGM. By accumulating the aggregate flux of line emissions such as [C II]$_{158\mu\rm m}$, CO, and many more from numerous sources, line-intensity mapping considers the contribution of flux from even the faintest sources; this technique eliminates the need to resolve individual luminous sources. Therefore, it can probe larger cosmological volumes in comparatively lesser observational time than conventional galaxy surveys such as ALMA, JWST, and many more. One can cross-correlate multiple tracers of line-intensity mapping, which is expected to mitigate contamination in the cross-correlation signal from systematic effects and interlopers. Our study shows that the line-of-sight anisotropies, such as the light-cone effect, significantly affect the cross-power spectrum between the [C II]$_{158\mu\rm m}$ signal from the galaxies and the [H I]$_{21\rm cm}$ signal from the IGM at large scales ($k \sim 0.2\, \rm Mpc^{-1}$), with the impact reaching up to 20 percent, depending on the reionization history. Therefore, one must consider the light-cone effect to interpret the LIM cross-power spectrum. On the other hand, the halo-mass dependent line-luminosity scatter is expected to impact the LIM signal fluctuations and, hence, its summary statistics, such as the power spectrum. Using results from hydrodynamical simulations, we show that the large-scale [C II]$_{158\mu\rm m}$ power spectrum is significantly affected due to the line luminosity scatter. Also, this impact can be robustly modelled using the most probable fit as the relationship between the [C II]$_{158\mu\rm m}$ line luminosity and its host halo mass. Similarly, the variability in the star-formation rate of the galaxies of the same halo mass can affect the ionizing luminosity of these reionizing sources and leave imprints on the reionization of the IGM. Using [H I]$_{21\rm cm}$ bispectrum, we find that for the small scales ($k_1 \sim 2.55\, \rm Mpc^{-1}$), there is a significant impact due to this astrophysical scatter ($|\langle\Delta B\rangle/B_{\text{no-scatter}}| \gtrsim 20$ percent). At a neutral fraction of $\overline{x}_{\rm HI} \sim 0.8$, this impact can be as high as $\sim 100$ percent. The impact of astrophysical scatter on the small scales is also statistically significant ($\gtrsim 5\sigma$ at $\overline{x}_{\rm HI} \gtrsim 0.8$), whereas on the large scales, it is dominated by statistical noise. In the most optimistic scenario, SKA1-Low might be able to detect the small-scale equilateral bispectrum with good detection significance ($\sim 3\sigma$ and $\sim 5\sigma$ at $\overline{x}_{\rm HI} \sim 0.8$ and 0.9 respectively), where the impact of astrophysical scatter is expected to be statistically significant and sufficient in magnitude. On the other hand, we explore the prospects of detecting the CO-[H I]$_{21\rm cm}$ cross-power spectrum from the Epoch of Reionization at $z \sim 7.2$ considering AARTFAAC and COMAP surveys. Our preliminary results suggest that the cross-correlation signal might be detected in the most optimistic scenarios of CO emission from the reionizing galaxies. However, it also needs to be investigated at the lower redshifts, where one might achieve better detection significance of the cross-correlation signal.

Speaker: Chandra Shekhar Murmu (Indian Institute of Technology Indore)

16:00 The impact of the H I reionization timeline on Ly$\alpha$ forest statistics

The reionization history of the intergalactic medium (IGM) is not perfectly constrained, although the midpoint is generally accepted to occur near $z\sim8$ and the whole process is likely concluded by $z\sim5.3$. The evolution of the neutral hydrogen fraction $x_\mathrm{HI}$ with redshift is being converged upon through a combination of many observational probes, but one that is particularly sensitive to the end stages at $z<6$ is the Lyman alpha forest (LAF). Metrics of the LAF, including the 1D flux power spectrum, autocorrelation function, and effective optical depth distributions have already been used to constrain the timeline, but with a more systematic approach to the investigation, the uncertainties may be reduced even further. To facilitate generation of improved late-stage constraints, we have developed a framework for simulating reionization using the Nyx cosmological hydrodynamics code paired with the AMBER (Trac et al. 2022) model for inhomogeneous reionization. AMBER parametrizes the reionization history in terms of a duration, midpoint, and asymmetry, which are paired along with Nyx-specific parameters such as the ionization front (i-front) heating and UVB details. To enable a more fine-grained evaluation of the parameters, we are creating neural network emulators of several LAF metrics based on a grid of these simulations, starting with the LAF power spectrum. Exploring the general trends, we find that the pressure smoothing scale within the IGM is strongly correlated with the adiabatic index of the temperature-density relation. We find that while models with 20,000 K i-front heating are better able to reproduce the shape of the observed $z=5$ 1D flux power spectrum than colder ones, they fail to match the highest wavenumbers. Further, we note that longer and more symmetric histories with greater i-front heating produce higher values of the autocorrelation functions and wider optical depth distributions, both of which are better matches to the observed LAF metrics than shorter, asymmetric, and cold i-front histories.

Speaker: Caitlin Doughty (Leiden Observatory)

16:15 Simulation-based inference and generative machine learning for LIM cosmology

Multiple-line intensity mappings can trace large-scale structure from now up to reionization, when the first galaxies formed. LIM 3D tomography provides valuable insights into cosmological structure growth, the intergalactic medium, and the properties and environment of ionizing sources. These mappings need updated modeling and inference methods for LIM cosmology because of the large scale and temporal range, highly non-Gaussian measured LIM signals, and large quantity of data. Our group in Heidelberg has developed a framework to both create mock simulations of large-scale tomographic LIM mappings (such as Lyman-alpha, H-alpha, and 21cm) using fast semi-numerical (approx. hydrodynamical) simulations coupled with scaling relations and to apply simulation-based inference to LIM tomography. As highlighted with 21cm tomography, in this talk I will show that a convolutional summary network coupled to a conditional invertible network through a physics-inspired latent representation performs optimal and extremely fast posterior inference of astrophysical and cosmological parameters. Our method's ability to detect non-Gaussian information makes it a viable alternative to power spectra as a summary statistic for LIM inference. We further investigate the generation of simulated LIM volumes using advanced machine learning, in particular generative networks, and discuss the potential synergy with our previous tomographic mocks.

Speaker: Lara Alegre (University of Heidelberg)

CosmicDawn2024_LA.pdf

16:30 21-cm integrated Sachs-Wolfe effect as a probe of the global 21-cm background

We propose a novel method to probe the spectral evolution of the global 21-cm background. Late-time evolution of our universe drives relic backgrounds to be correlated with low-redshift density tracers. This phenomena, the integrated Sachs-Wolfe effect (ISW), has been measured extensively for the cosmic microwave background (CMB) but never for the 21-cm background. We show that the 21-cm background also experiences ISW, and due to its dependence on the (1) sky-averaged amplituded and (2) the spectral shape, measuring the 21-cm ISW in conjunction with the CMB ISW can directly probe the time evolution of the global 21-cm background. A huge benefit over the single-element detectors such as EDGES and SARAS is that the 21-cm ISW is blind to the Milky Way contamination, thus naturally mitigating the ambiguity caused by the foreground removal process. We also show that, as long as SKA1-LOW conducts a full-sky observation for a few 1000s of hours, a new constraint on the evolution of the global 21-cm background can be made with high significance.

Speaker: Kyungjin Ahn (Chosun University)

16:45 Discussion

Speakers: Joakim Rosdahl, Yuichi Harikane

Friday, 28 June

09:00 → 09:15 Information from the LOC

09:15 → 12:00 Session 8

09:15 A post-reionization model for emerging Lyman-Alpha through UV observables

The Epoch of Reionization (EoR) marks the transition from a neutral to an ionized intergalactic medium (IGM). A key method to trace the rate of this transition is Lyman Alpha (Ly𝜶) emission from galaxies. The evolution of the IGM’s HI fraction can be inferred from Ly𝜶 observations due to its resonant nature and the damping of its emission. But to understand the effect of inter-galactic HI over Ly𝜶 we must first answer: How does Ly𝜶 look right as it emerges from a galaxy?
We present a new high-resolution spectra dataset from MMT/Binospec, accompanied by NIRCam/JWST slitless spectra coverage from the FRESCO collaboration. We provide detailed Lya profile properties of 70 galaxies at the edge of the EoR (z~5-6.5) plus 200 non-emitting Lyman-Break selected sources. The addition of high-redshift H𝜶 detections allow us to cover the systemic redshift and new properties such as Ly𝜶 escape fraction and velocity offsets.
We produce an empirical model in a Bayesian framework, used to infer the probability density function of Ly𝜶 equivalent width and escape fraction, based on UV magnitude and slope. We accompany this model by probability distributions of line emission broadness as trackers of NHI and dynamical mass, as well as Lya asymmetry results allowed by our high-resolution dataset.
In this talk I will present our empirical models for emerging Lya and discuss possible correlations originating from the ISM of young galaxies, as well as the evolution of the velocity offset of Lya with halo mass, and its implications for the search of large ionized bubbles.

Speaker: Gonzalo Prieto-Lyon (DAWN - Niels Bohr Institute - University of Copenhagen)

09:30 Spectroscopic characterization of galaxies at 9.5 < z < 12.5 in the Abell2744 field

The quest to discover galaxies beyond redshift z = 9 and probe their properties has been a key motivation for the development of the James Webb Space Telescope (JWST). Early spectroscopic campaigns using JWST NIRSpec have already validated photometric selections and allowed us to probe the physical conditions of galaxies in the early universe. Intriguingly, NIRSpec follow-up observations of high-redshift objects have suggested a surprisingly high incidence of AGNs, with candidates reaching z ≈ 10 and beyond. In this context, I will present the spectroscopic characterization of seven galaxies within the redshift range 9.5 < z < 12.5, as part of Program GO-3073 (PI M. Castellano) targeting the GLASS field with JWST NIRSpec/PRISM. All these targets were initially identified as robust z > 9 candidates. I will also discuss the high number density within the Abell 2744 field, as revealed by spectroscopic observations, and consider these findings in light of recent UV luminosity functions at z = 10.

Speaker: Lorenzo Napolitano (Sapienza Università di Roma)

09:45 Probing the connection between IGM transmission and galaxies during cosmic reionization

During cosmic reionization, ionized regions gradually grew and overlapped in the IGM. Understanding when and how reionization happened is crucial for studying the early structure formation and the properties of first galaxies in the Universe. At z>5.5, the observed IGM optical depth shows a significant scatter, indicating an inhomogeneous reionization process. However, the nature of the inhomogeneous reionization, whether it is dominated by the large-scale fluctuations in the ionizing background or in the IGM temperature, remains unclear. ASPIRE is a JWST cycle 1 program that has spectroscopically identified more than 400 [OIII] emitters at z>5 in 25 quasar fields at z>6.5. These [OIII] emitters provide independent and sensitive tracers of large-scale galaxy density in 25 quasar fields. Combined with deep ground-based optical spectroscopy of 25 ASPIRE quasars, covering Lyman alpha forests from z~5.3 to z~6.3, ASPIRE program provides the current largest sample for IGM-galaxy connection studies during cosmic reionization. We present the preliminary relation between IGM optical depth and galaxy overdensities at z>5.5 using ASPIRE program. We will discuss their constraints on reionization models and implications on the inhomogeneity of reionization process.

Speaker: Xiangyu Jin (The University of Arizona)

10:00 Unveiling the properties and evolution of extremely overdense regions in the very early Universe

Extremely overdense regions in the early Universe are likely engines for the production of hydrogen ionising radiation in the early Universe. The copious number of UV-faint galaxies and the idealistic environment for driving AGN activity means the most overdense environments, protoclusters, should drive some of the first ionised bubbles. As such, characterising the properties of these protoclusters is crucial in understanding their role in the Epoch of Reionisation. We present NIRSpec and NIRCam/Grism spectroscopic observations of galaxies resident within three protoclusters at z=7.66-10.6. One protocluster shows evidence of a dusty, metal-enriched and evolved core and low metallicity, high temperature, young galaxies in the outer regions. Such constraints on the build-up of dust and metals in overdense regions are unsurprising in the context of hierarchical growth, however the detection of Lya emission from one of the galaxies on the outskirts of the protocluster allows for constraints on the formation of an ionised bubble. We also provide the first spectroscopic confirmation of a z=7.66 protocluster, the first such structure identified by JWST in its first weeks of operation. This NIRCam Grism data allows for a complete sample of bright emission-line galaxies in the structure as well as strong constraints on the distribution of star formation. Finally, we provide evidence that one of the brightest z>10 galaxies, GNz11, lies at the centre of an overdense structure of UV-faint Lya-emitters, likely representing an early protocluster core. Moreover, we will discuss the differences in the evolutionary phases of these three protoclusters that likely results in their differing Lya transmission. Finally, we compare these results with state-of-the-art simulations to understand the interplay between protocluster-resident galaxy properties and their environments. JWST has opened a new era in understanding the properties and evolution of these most overdense structures in the early Universe. We exploit this capability to unveil our most in-depth understanding of protoclusters at high redshifts.

Speaker: Callum Witten (Institute of Astronomy, University of Cambridge)

10:15 Implications for early star formation and ionized bubble growth from the JADES UV luminosity function at z~9-15

The rest-frame UV luminosity function (UV LF) and evolution thereof is a crucial component of understanding star formation in the early universe, and how star-forming galaxies contributed to cosmic reionization. Prior to JWST, the rate of decline of the UV LF at z > 8 was unclear, but early analyses of JWST data have suggested the presence of an unexpectedly large abundance of luminous galaxies at unexpectedly high redshifts, potentially pointing to previously unknown factors governing early star formation (e.g. an evolving star formation efficiency, a top-heavy stellar initial mass function, stochastic star formation histories, etc.). In this talk, I will present the UV LF at z~9-15 derived from ~250 galaxies identified over the full 125 square arcminutes of JADES imaging, providing a view of the redshift evolution of the UV LF and cosmic UV luminosity density down to M_UV~-16.5 over ~300 million years of cosmic time. I will first discuss implications for early star formation of the UV LF evolution found in the JADES fields. Then, I will consider implications of the z~9-15 UV luminosity density for the reionization timeline and early growth of ionized bubbles, especially in the context of the possible existence of large, pMpc-scale ionized bubbles as early as z~9.

Speaker: Lily Whitler (University of Arizona)

10:30 Morning coffee/tea break

11:00 Measuring the timing and topology of the Epoch of Reionization

Despite it being a fundamental milestone in our Universe’s evolution, the epoch of reionization (EoR) remains poorly understood. Luckily, recent years have witnessed a large increase both in observational datasets probing the EoR as well as sophisticated theoretical frameworks used to interpret this data. We introduce two novel, forward-modeling, implicit-likelihood inference frameworks designed to infer both the timing and the topology of the EoR. We demonstrate how recent high-redshift QSO spectra from XQR30+, combined with observed UV luminosity functions, already constrain the bulk of the EoR history to within \Delta z ~ 0.1, in a way that is robust to the choice of astrophysical model and without the inclusion of ad-hoc, “tunable" parameters. The topology of the EoR however remains a mystery. This topology tells us which sources reionized the Universe, and allows us to connect the growth of cosmic HII regions to the galaxies they contain. We showcase a novel framework to infer the local HII region surrounding groups of galaxies. We forward model the Lyman alpha spectra of galaxies as would be observed by NIRSpec on JWST, sampling all relevant sources of uncertainty, and taking into account each galaxy’s relative location in the local EoR topology. We find that observed galaxy number densities of ~0.003 cMpc^-3 are sufficient to estimate the size and position of the local HII region at percent level accuracy. Such number densities are well within the reach of future JWST surveys.

Speaker: Andrei Mesinger (Scuola Normale Superiore)

11:15 Behaviour of the Ly-$\alpha$ damping wings statistics as a function of reionization topology

The intergalactic medium damping wing signatures of the highest redshift quasars have been an effective tool in deciphering the epoch of reionization. However, only a handful have been discovered so far above redshift 7. In the near future, the Euclid mission will enable the discovery of an order of magnitude more quasars for damping wing studies. In light of this impending flood of new quasars, it is worth examining whether they can teach us more about reionization than just its history. That is, the topology of reionization should influence the shape and sightline-to-sightline variations of the quasar-damping wing signal. In our work, we have studied how the astrophysical parameters that govern the reionization topology affect the statistics of damping wings. We use 21cmFAST to generate a suite of patchy reionization models in large cosmological volumes across a wide range of astrophysical models for the sources of ionizing photons and compute the mean and scatter of the expected damping wing signal. We examine the interplay between the neutral fraction, quasar lifetime, quasar host halo mass, and source parameters that affect the reionization topology such as the minimum dark matter halo mass that supports star formation. Furthermore, we systematically explore the convergence properties of damping wing signals across diverse source models and multiple simulation volumes. We present a suite of models to assess both the ability of future quasar samples to constrain astrophysical model parameters and the additional systematic uncertainty on the neutral fraction incurred when they are fixed to a single fiducial value. We will make the suite of large-volume simulation boxes and damping wing spectra publicly available.

Speaker: Yash Mohan Sharma (Max Planck Institute for Astronomy (MPIA))

Sharma_Cosmic_Dawn_ppt.pptx

11:30 Mapping reionization with JWST observations

Measuring the size distribution of ionized bubbles is key to understanding the physical properties of early galaxies, including those too faint for even JWST to detect directly. How do we measure these ionized bubbles? How can we infer the properties of reionizing galaxies from the measured bubble sizes? Excitingly, JWST’s excellent capability of detecting Lyman-alpha emission lines from galaxies and spectroscopically confirming galaxies, enables us to map the galaxy and ionization field in 3D for the first time at z>6. To test how we can use the observed galaxy Lya emissions to map ionized bubbles and learn about ionizing sources, we forward model galaxies’ Lya emission in realistic reionizing intergalactic medium simulations. We have developed a method to infer the ionized regions from galaxies using their Lya emission, for the first time combining observations of multiple galaxies to map out ionized structures. In this talk, I will present the Lya emission-bubble mapping method. I will also discuss the requirements and strategies of mapping ionized bubbles with JWST and future facilities.

Speaker: Ting-Yi Lu (Cosmic Dawn Center, University of Copenhagen)

11:45 Quasars and Ionized Bubbles in Lyman-alpha forest Calibrated Radiative Transfer Simulations of Reionization

The influx of data from JWST, coupled with advancements in radiative transfer simulations, is improving our understanding of cosmic reionization. Recent findings suggest a late and rapid reionization model leads to the best fit to current observations. However, the identification of high-redshift AGNs and LAEs challenge these models, necessitating a reevaluation of AGNs and galaxies' roles in this epoch. In this talk, I will present new results from fast GPU-accelerated radiative transfer simulations with the code ATON utilizing the SHERWOOD suite of hydrodynamical simulations. We have augmented ATON to include helium, expand across multiple frequency bins, and simulate the contribution of both galaxies and AGN to the ionizing emissivity across a broad spectrum of reionization histories extending from $z=15-20$ to the end of reionization at $z\approx5$. Our simulations are all calibrated to reproduce the latest Lyman-alpha forest opacity observations to ensure an accurate end to reionization. The presentation will centre on the incorporation and properties of quasars within these simulations, their influence on ionizing emissivity and reionization history. Additionally, the talk will delve into the morphology of ionized regions, as well as the thermal state of the intergalactic medium in QSO assisted models of reionization, with a view to interpret JWST observations of reionization-epoch galaxies and AGN.

Speaker: Shikhar Asthana (University of Cambridge)

12:00 → 13:15 Buffet lunch

13:15 → 16:50 Session 9

13:15 Observational Benchmarks of Early Galaxy Evolution from Extreme Emission Line Galaxies

Speaker: Danielle Berg

13:45 Probing stellar feedback and ionising escape with accurate emission line modelling

JWST has provided an abundance of spectroscopic and photometric data during the Epoch of Reionisation and beyond. However, understanding the underlying physics of galaxies during these early epochs remains a formidable challenge as we rely on biased observational methodologies such as SED fitting to interpret the data and compare to simulations. I will present results from zoom-in re-simulations of the THESAN box, which use the state-of-the-art AREPO-RT radiation hydrodynamics code with a variation of the SMUGGLE galaxy formation model. I will demonstrate our methodology to fully forward model observations with the Monte Carlo radiative transfer code COLT, utilising a novel post-processing approach to complement the rich on-the-fly physics and obtain reliable continuum, collisional line, and recombination line maps. I will show how we compare emission line ratios from our simulated galaxies to deep JWST spectroscopic surveys such as JADES to robustly investigate star formation and feedback models, including early stellar and SN feedback, as well as explore the evolution of the IMF. Furthermore, I will show how different stellar feedback models and modes of quenching (e.g. starvation, internal feedback, external radiative feedback) relate to the ionising escape fractions of galaxies, and how this can affect our understanding of which galaxies are primarily responsible for reionising the universe.

Speaker: Will McClymont (University of Cambridge)

14:00 Deciphering Lyman Escape in Anisotropic Gas Configurations

With the James Web Telescope (JWST) offering a prime opportunity to observe Lyman-alpha (Lya) in the high-redshift Universe, understanding the information embedded in Lya observables and its relation to ionizing radiation has become a crucial task in the study of the Epoch of Reionization. Since Lya is sensitive to gas density, dust, and fragmentation, it provides vital information about the interstellar medium (ISM) and the circumgalactic medium (CGM). Deciphering this information remains a complex challenge due to Lya resonant nature and complex radiative transfer effects. Current isotropic radiative transfer models reproduce the line profile but lack clarity on how fitted parameters relate to actual physical conditions. For more accurate modeling, we must consider key features necessary for Lya and Lyman-Continuum (LyC) radiation escape: astrophysical gas is anisotropic and multiphase.
In this talk, I will discuss the results of Monte Carlo radiative transfer simulations of Lya performed on an anisotropic setting: a slab with an empty channel carved in the neutral gas distribution. Regardless of the setup simplicity, the results are counterintuitive and far from simple. Given the large line-center optical depth of the slab, one would expect that most Lya photons escape through the empty channel. Our numerical results show, in contrast, that only a fraction of photons escape. To explain this dilemma, we developed an analytical model describing Lya photons' general scattering and transmission behavior. Our findings show a much greater number of scatterings per reflection, leading to an overall much greater transmission probability through the slab. These results imply that Lya photons are surprisingly not as biased to the 'path of least resistance' and can trace an averaged hydrogen distribution rather than only low-column densities. Time provided, I will also discuss the implications of this result when interpreting the observed Lya profiles, specifically, when used as a tracer for ionizing photons.

Speaker: Silvia Almada Monter (Max Planck for Astrophysics)

14:15 Mapping the Influence of Cool Outflows on LyC Escape in High Redshift Analogs

Constraining the Lyman continuum (LyC) escape fraction, fesc, at high redshifts is essential for mapping the ionizing emissivity of early galaxies during Reionization, but attenuation by the neutral IGM renders direct measurements impossible.  As such, various diagnostics, such as [OIII]/[OII] ratios, UV-slope, etc., have been studied in local high-redshift analogs to develop techniques to infer fesc. While no single diagnostic has prevailed as a feasible model to constrain fesc at high-z, the consensus of these studies concludes that line of sight (LOS) diagnostics are essential for measuring fesc. Understanding the relationship between galaxy properties and the LOS geometry of neutral gas and dust content of the ISM/CGM is therefore paramount to understanding the physics which reionized the universe. In this talk, we present on our work to study the influence of cool outflows on LyC escape in a sample of confirmed LyC leakers and non-leakers taken from the low-z LyC survey (LzLCS). Specifically, we perform radiation transfer modeling of the Mg II 2796A, 2803A doublet in high resolution follow-up spectra of the LzLCS galaxies to map the cool phase of their outflows. Assuming Mg+ traces neutral hydrogen, we infer the neutral hydrogen distribution to predict fesc with cloudy models. Our predictions often overestimate fesc, suggesting that moderately optically thin gas significantly influences fesc in addition to the binary optically thick and thin LOS distributions typically assumed in these types of models. Lastly, we present on our results pertaining to how outflow properties, such as mass outflow rate and velocity dispersion, impact fesc in the compact + high star formation rate surface density regime, where the strongest LyC emitters lie in LzLCS.

Speaker: Cody Carr (Zhejiang University)

14:30 Escape the Block: Mapping pathways of ionizing leakage at in high detail with JWST

Recent years have seen mounting evidence that the dominant contribution of
Lyman-Continuum photons driving cosmic Reionization has come from hot, massive stars in the first galaxies. How these photons have escaped the shroud of HI in their galaxies of origin to ionize the IGM is still debated and studied intensely. Many mechanisms have been suggested that can create the necessary escape channels, but testing these hypotheses is challenging. It requires mapping the production and escape sites of LyC photons, as well as a mapping of ISM properties on scales spanning from individual HII regions to galaxy-spanning, large scale outflows.

Such studies are not possible to carry out at low redshifts with current
facilities; the only current instrument capable of observing ionizing photons at low redshifts is the Cosmic Origins Spectrograph onboard HST, which has an aperture size of 2.5", limiting the spatial precision of these observations to a few hundred parsec in the nearest known LCEs. At higher redshifts, where the observations of LyC is less technically challenging, the distance becomes the limiting factor to the attainable spatial detail. Ironically, the best prospects for this kind of study are found at cosmological distances, where the probability of strong gravitational lensing is grows large enough to allow us to catch strongly lensed leakers in large enough magnification to enable this challenging kind of study.

In this talk, I will present detailed JWST/NIRCam and NIRSpec Integral Field Spectroscopic observations of physical ISM properties of the ISM of the gravitationally lensed z=2.4 Sunburst Arc. This galaxy is host to a massive, bright, 30 Dor-like young cluster complex which spills copious amounts of LyC emission into the IGM through a line of sight extremely sparse in neutral Hydrogen. Fortunate alignments of the galaxy and the lensing cluster has led to as many as 12 lensed images of this cluster complex, with magnifications allowing us to probe ISM properties on scale locally down to $\lesssim 10$ pc. The combination of the strong magnification and the capabilities of JWST has enabled the mapping of a wealth of ISM properties in the galaxy such as kinematics, ionization structure, and chemical abundance patterns. Together with previous imaging in LyC wavelengths with HST, this dataset makes up the first of its kind, allowing us to map both ionizing production, escape and ISM properties at scales down to individual HII regions. This allows us to spatially correlate physical ISM properties and proposed secondary tracers of LyC escape to high accuracy, allowing unprecedented tests of both escape mechanisms and secondary observational tracers.

Speaker: T. Emil Rivera-Thorsen (Stockholm University)

14:45 Afternoon coffee / tea break

15:15 Stacking PANCAKEZ: sPectroscopic Analysis with NirspeC stAcKs in the Epoch of reioniZation

Young star-forming galaxies are believed to be the main contributors to the process of cosmic reionization which completed roughly $1$ billion years after the big bang, at a redshift $z\sim6$. However, it is unclear whether such galaxies actually had sufficient ionizing escape fractions, as direct measurements are impossible due to the neutral intergalactic medium at this epoch. Indirect measurements of the HI gas covering fraction via low-ionization interstellar (LIS) absorption lines offers a promising means to constrain the escape fraction. While reionization-era galaxies are typically too faint for absorption line measurements, we use stacking analysis of JWST/NIRSpec data to determine the LIS absorption properties of galaxies at $z>6$, representing the first such measurements in the epoch of reionization. We perform a comprehensive spectral stacking analysis on $> 50$ galaxies at mean $z=7.48$ to measure equivalent widths (EWs) of LIS absorption features as well as Ly$\alpha$. We infer constraints on evolution in the covering fraction of HI, and hence the ionizing escape fraction, based on the LIS measurements from our stacks. Our results also reveal the unambiguous presence of strong outflows in these early galaxies, with composite spectra exhibiting blueshifted LIS features with redshifted Ly$\alpha$ emission, which has important implications for the gas content and star formation burstiness in early galaxies. I will also discuss complementary measurements of the interstellar gas content of faint $z\sim7$ galaxies from ALMA observations. Collectively, our results constrain both the escape of ionizing photons and the nature of feedback at $z>6$, giving insight into the properties of early galaxies and their role in cosmic reionization.

Speaker: Kelsey Glazer (University of California, Davis)

15:30 Inferring the Distribution of the Ionising Photon Escape Fraction

The reionisation of the IGM was driven by the first galaxies. However, the nature of these galaxies remains uncertain. This includes how many stars form in a galaxy, what types of stars form and what fraction of the ionising photons produced in the galaxy escape into the IGM. In particular, the ionising photon escape fraction is poorly constrained observationally and we lack a clear picture of which physical processes determine how ionising photons escape.
The distribution of escape fractions in galaxy samples can constrain which leakage mechanisms dominate. However, until now we only have average escape fraction estimates from galaxy samples or direct measurements of the escape fraction from individual galaxies. For 4 physically motivated models of the escape fraction distribution, we have used hierarchical Bayesian inference to constrain the escape fraction distribution from a sample of reionisation-era analogue ($3 In this talk, I will present the inferred escape fraction distributions, discuss which of our models best describes the observed data and compare the best-fitting model to simulations. I will discuss what drives ionising photon escape and the implications on our understanding of reionisation.

Speaker: Kimi Cardoso Kreilgaard (The Cosmic Dawn Center, Niels Bohr Institute, University of Copenhagen)

15:45 Physical properties of extreme emission-line galaxies at z~4-9 from the JWST CEERS survey

Extreme emission line galaxies (EELGs) are typically characterized by high equivalent widths (EWs) which are driven by elevated specific star formation rates (sSFR) in low-mass galaxies with subsolar metallicities and little dust. Such extreme systems are exceedingly rare in the local universe, but the number density of EELGs increases with redshift. Such starburst galaxies are currently presumed to be the main drivers of hydrogen reionization over 5.5<z<15, which motivates the searches for high-z EELGs. In this talk, I will present the study of the physical properties of a sample of ~1000 EELGs at 4<z<9 photometrically selected from the CEERS survey using JWST/NIRCam. We create synthetic NIRCam observations of EELGs using empirical templates based on ~2000 local metal-poor starbursts (MPS) to select EELGs based on color-color criteria. We study their properties based on SED fitting and flux excess in the photometric filters. We consider a delayed-tau model for the star formation history and find EELGs are young with a mean value of the time after the onset of star formation of 93Myr. We find that they have similar line ratios to local MPS with high log([OIII]/Hbeta)>0.4 which indicates that star formation may be the dominant source of ionization. We find an increase of EW([OIII]+Hbeta) with sSFR and SFR surface density, and a decrease with age and stellar mass. The sample of EELGs can reach high SFR surface density and high O32>5 which indicates they may have the conditions to facilitate the escape of ionizing photons.

Speaker: Mario Llerena (INAF-OAR)

16:00 Discussion and closing remarks

Speakers: Danielle Berg, Organizers