A search for H  <scp>i</scp> absorption in distant star-forming galaxies with ASKAP-FLASH - II. Direct observations and stacking of 21 cm line

Abstract We present results of a search for associated and intervening H i 21 cm absorption in star-forming galaxies and a control sample of radio galaxies at 0.4 &amp;lt; z &amp;lt; 1.0, using ASKAP’s FLASH H i 21 cm absorption survey. We report the detection of a new H i 21 cm absorption line in radio galaxy NVSS J214954-004657 at zHI = 0.863. Additionally, we present results of H i 21 cm stacking for associated and intervening star-forming and radio galaxy catalogues. For the first time at these redshifts, we report tentative detections of stacked H i 21 cm absorption for associated galaxies when stacked in order of descending radio source brightness, and show that these tentative detections become buried as optical depth increases as progressively weaker sources are co-added into the stack. No stacked signal is detected when co-adding intervening galaxy spectra. We speculate that the ability to detect a stacked H i 21 cm detection is dependent on the background source optical depth limits, as well as source covering factors. Covering factor has a greater impact when stacking intervening systems, and can be considered to effectively weaken the observed flux density of a radio source. We find no significant difference in detection rate for associated H i 21 cm absorption in star-forming galaxies versus the general radio-source population at the same redshift. Although detection rates are low, from a meta-analysis of H i 21 cm surveys we find that 1 detection is expected given the survey parameters.