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

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.

Exploring Neutral Hydrogen in Radio MOlecular Hydrogen Emission Galaxies (MOHEGs) and Prospects with the SKA

“…The H I 21 cm absorption lines are powerful tracers of cold gas in the central regions (at parsec scales), close to the central black hole of galaxies and up to tens of kiloparsec. They trace interaction and mergers into galaxy groups as well as the intervening neutral gas clouds in foreground galaxies, tails, and filaments (Morganti & Osterloo 2018;Dutta et al 2022). H I absorption in radio sources is mostly detected against the central regions, allowing us to probe the gas properties in the inner regions of the sources and the AGN structures.…”

“…Galaxies such as radio MOHEGs are expected to have lower H I column densities, which make it difficult to detect them in emission surveys with current radio instruments. H I absorption studies against bright radio continuum background can track the neutral hydrogen in galaxies, which otherwise could not be seen in H I emission observations due to the constraints of the achievable sensitivities at larger distances (Morganti & Osterloo 2018;Dutta et al 2022). H I surveys offer substantial advantages to trace the star formation and galaxy evolution from nearby to the distant universe due to their independence of the effects of extinction by dust and color.…”

“…Gas-rich galaxy disks hold extended atomic gas in their disk regions, which is susceptible to distortions during tidal interactions. The effects of these activities can be studied through observations of the H I 21 cm transition (Yun et al 1994;Noordermeer et al 2005;Chung et al 2009;Brown et al 2017;Dutta et al 2022). Additionally, observations of atomic gas outflows in radio galaxies (RG) suggest that AGN radio jet feedback has a galaxy-scale impact on the host ISM (Morganti et al 2003(Morganti et al , 2005Guillard et al 2012;Schulz et al 2021).…”

“…A threshold neutral hydrogen column density of N(H I) ∼5 × 10 20 cm −2 is required to begin with the formation of H 2 , which helps to shield the gas against radiation from UV photons (Stecher & Williams 1967;Hollenbach et al 1971;Federman et al 1979;Kanekar et al 2011). Unlike the H I emission lines, where the flux attenuates with distance, H I absorption features are independent of the distance of the source; rather, they primarily depend on the strength of the background radio continuum, the covering fraction ( f c ) of the source, and the spin temperature T s (K) (Dutta et al 2022). Hence, the H I 21 cm absorption line studies can complement the emission line surveys to trace the evolution of the atomic gas component in and around galaxies.…”