Probing galaxy evolution through H i 21-cm emission and absorption: current status and prospects with square kilometre array

One of the major science goals of square kilometre array (SKA) is to understand the role played by atomic hydrogen (H i ) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of H i gas associated with galaxies. In this paper, we review our current understanding of H i gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the H i science that will be possible with SKA and its precursors and pathfinders, i.e., H i 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.