Neutral hydrogen (HI) is the most abundant gaseous element in the interstellar medium of galaxies, and acts as a key reservoir for star formation, particularly in its cold phase. Probing the distribution of HI is hence critical for understanding the evolution of galaxies. The HI 21-cm hyperfine transition is an excellent means of tracing neutral gas in galaxies from low to high redshifts, and the neutral gas can be detected through both HI 21-cm emission and absorption. Aditya et al. report an ASKAP search for associated HI 21-cm absorption against bright radio sources from the Molonglo Reference Catalogue (MRC) 1-Jy sample. The search uses pilot survey data from the ASKAP First Large Absorption Survey in HI (FLASH) covering the redshift range 0.42 < z < 1.00. From a sample of 62 MRC 1-Jy radio galaxies and quasars in this redshift range they report three new detections of associated HI 21-cm absorption.

The figure above shows the detection of a narrow HI 21-cm absorption line towards MRC 2216-281, where the line peak has a redshift of z=0.626. The grey shaded region represents 5𝜎 noise level. The AGN redshift previously reported in the literature, of z=0.657 ± 0.050, has a large uncertainty which means that it is not possible to assess whether the detected absorption is associated with the MRC galaxy, or if it is arising from a lower-redshift gas cloud that is intervening our line of sight.