A search for HI absorption at intermediate redshifts in star-forming galaxies with ASKAP-FLASH

There is a marked decline in the global star formation rate from its peak at z = 2 to present day (z = 0), of around a magnitude. This phenomenon, ‘cosmic downsizing’, has been studied for around two decades, but the underlying cause is not understood, although the evolution of neutral hydrogen (H i) over time is suspected to be involved. H i forms the reservoir of material for star formation in galaxies, and can be studied at z < 0.4 via the characteristic 21 cm emission line. However, at z > 0.4 this line is intrinsically too faint to be observed. Using 21 cm absorption, which is instead limited by radio continuum source strength, we search for H i absorption in star-forming (SF) galaxies with ASKAP-FLASH at 0.4 < z < 1.0.For the first time, we show that stacked 21 cm signals can be detected, and find that the detection of the signal is based on optical depth and the strength of the radio sources that are co-added into the stacks. We detect two new 21 cm absorption signals - one confirmed in J214954550-00465630, and one tentative towards J144546661-01304141 - and investigate their optical depth and column densities, along with setting optical depth and column density limits for 36 other galaxies. From our samples of 25 SF and 23 radio galaxies, we find no significant difference in H i detection rates for bright SF galaxies (4.0+0.9 −1.6 per cent) compared to the general population of radio galaxies (4.3+1.9−1.4 per cent) at the same redshifts.The work presented here progresses the understanding of H i at 0.4 < z < 1.0, and develops a technique for future statistical 21 cm searches with next generation radio telescopes such as SKA, which will help further our understanding of the link between H i content evolution and cosmic downsizing.