Probing the epoch of reionization using synergies of line intensity mapping

Abstract

The epoch of reionization (EoR) remains a poorly understood cosmic era for the most part. Yet, efforts are still going on to probe and understand this epoch. We present a review of the latest developments in the techniques (especially line-intensity mapping) to study the EoR and try to highlight the contribution of the Indian community in this field. Line-emissions like [Hi]$_{\mathrm{21~cm}}$, Lyman-$\alpha$, [Cii]$_{\text {158}~\mu \text {m}}$ and their role as tracers in probing the EoR are discussed. While the [Hi]$_{\mathrm{21~cm}}$ is an excellent probe of the early inter-galactic medium (IGM), the others are mainly targeted to do an unresolved and large-scale survey of the reionizing sources. Techniques to model these signals include simulations and machine learning approaches along with the challenge to tackle foregrounds or interlopers. We also discuss synergy opportunities among various tracers that we mention. Synergy addresses different aspects of the problem, which is otherwise difficult or impossible to tackle. They include statistics like cross-power spectrum, cross-bispectrum and other techniques, such as follow-up studies. We present updates on the relevant experiments; these include upper limits on the [Hi]$_{\mathrm{21~cm}}$ power spectrum along with some highlights on high-redshift galaxy surveys. Finally, we highlight on what can be improved further within the community: applying machine learning and simulations based on hydrodynamic and radiative-transfer techniques. Next-generation experiments also need to be conceived to address issues, which are currently beyond our reach.