It is known that the rate at which stars are produced in galaxies has dropped by a factor of ten in the last 9 billion years (from z ~ 1.0). However, the gas content of galaxies - the fuel supply for star-formation - is only poorly constrained by observations during this period of time.
Quantifying the atomic neutral hydrogen gas (HI) content of galaxies using 21 cm emission from distant galaxies (z > 0.1) is difficult with current radio telescopes. However, the average HI content of galaxies at moderate redshifts can be measured by coadding the HI signal from multiple galaxies with known optical positions and redshifts. Our group has been pioneering this technique using the Giant Metrewave Radio Telescope (GMRT) and the Anglo-Australian Telescope (AAT).
I will present our current results for the average neutral hydrogen gas content in star forming field galaxies at a redshift 0.24 (look-back time of 2.8 Gyr) and the gas content of galaxies surrounding the galaxy cluster Abell 370 at a redshift of 0.37 (look-back time of 4.0 Gyr). I will also discuss the use of the HI coadding technique with the SKA pathfinders of MeerKAT and ASKAP and show that they can be used to quantify the average HI content of galaxies out to redshifts of 1.0.