Probing the Epoch of Reionization with line-intensity mapping

Line-intensity mapping (LIM) has emerged as a novel technique over the past decade 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]158𝜇m, CO, and many more from numerous sources, LIM instruments consider 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 also cross-correlate multiple LIM tracers, and mitigate contamination in the cross-correlation signal from systematic effects of the instruments and interlopers. Here, I will talk about the impact of line-of-sight anisotropies, such as the light-cone effect, on the [C II]158𝜇m ⨉ [H I]21cm cross-power spectrum, and why it is important to take this into account for proper interpretation of any summary statistic. On the other hand, the halo-mass dependent line-luminosity scatter in various line emissions from galaxies is expected to impact the LIM signal fluctuations and, hence, its summary statistics, such as the power spectrum. Using results from hydrodynamical simulations, our study showed that the large-scale power spectrum of [C II]158𝜇m line emission is significantly affected due to the line luminosity scatter. This impact on the large-scale [C II]158𝜇m LIM power spectrum can be robustly modelled using the most probable fit for the [C II]158𝜇m line luminosity and the host halo mass relationship. 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. Although this does not significantly affect the ionization power spectrum, we find that for the small scales (𝑘1 ~ 2.55 Mpc-1), the [H I]21cm bispectrum is significantly affected. The impact due to this astrophysical scatter on the [H I]21cm bispectrum lies in the range of 20 to 100 percent which is statistically significant (≳ 5σ at ≳ 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 (~ 3σ and ~ 5σ at ~ 0.8 and 0.9), where the impact of astrophysical scatter is expected to be statistically significant and sufficient in magnitude. I will also highlight the prospects of detecting the CO and [H I]21cm cross-power spectrum from the Epoch of  Reionization at z ~ 7.2 considering AARTFAAC and COMAP LIM 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.