Khagol Bulletin # 132 (Apr 2024) - ENG

| 08 | KHAG L | No. 132 - APRIL 2024 spin temperature of the transition. Further, HI 21 cm absorption has been used to investigate the filling factor of cold gas clouds in the ISM, the small-scale structures of the atomic gas using Very Long Base l i ne I nter feromet r y, the magnetic field via Zeeman splitting, and variations in the fundamental constants of physics like the electromagnetic fine structure constant, the proton-to-electron mass ratio, and the proton g-factor. Depending on which type of galaxy it is tracing, the HI 21 cm absorption line is classified as 'intervening' if it arises from a galaxy in between the background radio source and the observer, or as 'associated' if it arises from the galaxy of the background radio source itself. Associated HI 21 cmabsorption has been used to study the effect of feeding and feedback processes such as gas accretion and outflows in radio-loud Active Galactic Nuclei (AGN). The unprecedented sensitivity, sky coverage and spatial resolution of the Square Kilometre Array (SKA) will allow us to address a wide variety of open questions in the field of galaxy evolution using the HI 21 cm line. In preparation for SKA, a large number of surveys with SKA pathfinders and pre-cursors, e.g., uGMRT in India, ASKAP in Australia, and MeerKAT in South Africa, are being planned or on the way. Due to the intrinsic faintness of the HI 21 cm emission line, HI mapping of galaxies till date has been feasible only for nearby galaxies, up to redshift, z~0.2. Large HI 21 cm emission line surveys with the above telescopes (e.g., WALLABY, MIGHTEE-HI, MHONGOOSE, DINGO, LADUMA, etc.) are extending the scope of HI 21-cm emission studies of galaxies to higher redshifts (z~1) and larger statistical samples, by improving the observational capabilities in radio astronomy such as sensitivity, survey speed, and spatial resolution. When it What are the futureprospectsofHI 21cm studiesofgalaxies? comes to HI 21 cm absorption, the relatively small number of such absorbers detected to date (~200) hinders our ability to utilise these absorbers to trace the evolution of cold atomic gas and its interplay with galaxies across cosmic time. The main technical limitations affecting HI 21 cm absorption searches so far have been small bandwidths and restricted frequency ranges due to the presence of strong radio-frequency interference (RFI). The instantaneous large bandwidths, broad frequency coverage in low-RFI environments and high survey speed of the new radio telescopes are enabling large, blind and wide-area surveys of HI 21-cm absorption, e.g., MALSwithMeerKAT, FLASH with ASKAP, SHARP with APERTIF. These surveys will not just expand the sample sizes of HI 21-cm absorbers by over an order of magnitude, being blind surveys they will enable us to trace the cosmic evolution of cold atomic gas content in galaxies in a systematic and unbiased way using homogeneous samples.