Speaker
Description
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.
