The HI gas content of galaxies around Abell 370, a galaxy cluster at z=0.37

Abstract

We used observations from the Giant Metrewave Radio Telescope to measure the atomic hydrogen gas content of 324 galaxies around the galaxy cluster Abell 370 at a redshift of z = 0.37 (a look-back time of ^~4 billion years). The H I 21cm emission from these galaxies was measured by co-adding their signals using precise optical redshifts obtained with the Anglo-Australian Telescope. The average HI mass measured for all 324 galaxies is (6.6 ± 3.5) × 10⁹M_⊙, while the average HI mass measured for the 105 optically blue galaxies is (19.0 ± 6.5) #215; 10⁹M_⊙. The significant quantities of gas found around Abell 370 suggest that there has been substantial evolution in the gas content of galaxy clusters since redshift z = 0.37. The total amount of atomic hydrogen gas found around Abell 370 is up to approximately eight times more than that seen around the Coma cluster, a nearby galaxy cluster of similar size. Despite this higher gas content, Abell 370 shows the same trend as nearby clusters that galaxies close to the cluster core have lower HI gas content than galaxies further away where the galaxy density is lower. The optically blue galaxies contain the majority of the HI gas surrounding the cluster. However, there is evidence that the optically red galaxies contain appreciable quantities of HI gas within their central regions. The Abell 370 galaxies have HI mass-to-optical-light ratios similar to local galaxy samples and have the same correlation between their star formation rate and HI mass as found in the nearby galaxies. The average star formation rate derived from [OII] emission and from deredshifted 1.4 GHz radio continuum for the Abell 370 galaxies also follows the correlation found in the local Universe. The large amounts of HI gas found around the cluster can easily be consumed entirely by the observed star formation rate in the galaxies over ^~4 billion years (from z = 0.37) to the present day. Abell 370 appears set to evolve into a gas-poor system similar to galaxy clusters observed in the local Universe.