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 therefore 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 image above shows the HI 21-cm absorption spectrum toward the Compact Steep Spectrum source MRC 2156-245. The vertical dashed lines represent the redshifts determined from five different transitions in optical spectra, and the vertical dotted line at 0 km/s represents the reference for the average redshift, z = 0.862. The associated absorption feature can be seen near a velocity of 1000 km/s compared to the systemic recessional velocity of the source. “Associated” HI 21-cm absorption, defined to be within 3000 km/s of the galaxy redshift, is thought to arise from within the host galaxy. The grey shaded region represents 5𝜎 noise level — while no individual spectral channel exceeds the 5𝜎 significance level, the grouping of neighbouring channels clearly signifies an absorption feature.