Probing galaxies using neutral hydrogen gas

Evolution of galaxies through cosmic time is closely governed by gas flow processes occuring in and around them. Galaxies grow by accreting gas as fuel to sustain their star formation. Outflows due to star forming and nuclear activity in galaxies subsequently replenish the surrounding regions with gas and metals and can affect future star formation. Over the last few decades, it has become evident that a successful theory of galaxy formation and evolution requires us to understand the distribution and physical conditions of different gas phases in the gaseous haloes in which galaxies reside. Beyond the galactic haloes, the local environment plays a fundamental role in shaping the way galaxies evolve. Galaxy mergers and interactions in dense environments can funnel large quantities of gas to the central regions of galaxies, triggering intense bursts of star formation and fueling Active Galactic Nuclei (AGNs). Feedback from AGNs, both positive and negative, can in turn have a strong impact on the evolution of the host galaxies and the environment. The cold atomic hydrogen gas phase in particular is vital to understand the processes through which gas gets converted to stars in galaxies as well as feeding and feedback processes related with AGNs. HI 21-cm absorption is an excellent tool to probe the cold neutral gas phase across redshifts. Using extensive HI 21-cm absorption surveys, we have studied the distribution of cold gas around low redshift galaxies, physical conditions of cold gas in high redshift galaxies and gas flows in circumnuclear regions of galaxy mergers hosting AGNs. This talk will present key results from these studies and discuss briefly the prospects of taking such studies forward with next-generation large telescopes.