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,...
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...
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...
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...
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...
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...
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...
I will present a new determination of the evolving galaxy UV luminosity function (LF) over the redshift range $8.5
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...
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...
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...
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....
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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.
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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,...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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,...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
In this talk I will demonstrate the remarkable synergy of multi-tracer observations in the early Universe, presenting the first small survey from the Epoch of Reionization of the distribution and morphology of dust obscured star formation, along with resolved cool gas ((CII)158) kinematics. The ten-galaxy sample includes an optically dark 'ALMA only' galaxy and is otherwise comprised of the...
Understanding how galaxies evolved from turbulent compact systems to stable rotating discs remains a fundamental question in extragalactic astronomy. Studies of galaxy kinematics at z<3 have painted a picture where most star-forming galaxies exhibit disc-like kinematics that fuel an inside-out growth of galaxies and drive scaling relations between various properties such as star formation rate...
