Abstract:
One of the major science goals of square kilometre array (SKA) is to understand the role played by atomic hydrogen (HI) 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 HI gas associated with galaxies. In this paper, we review our current understanding of HI 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 HI science that will be possible with SKA and its precursors and pathfinders, i.e., HI 21-cm emission and absorption studies of galaxies from nearby to high redshifts that will trace various processes governing galaxy evolution.
