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 21 cm emission from these galaxies was measured by co-adding their signals using precise optical redshifts obtained with the Anglo-Australian Telescope. The average H i mass measured for all 324 galaxies is (6.6 ± 3.5) × 109 M⊙ , while the average H i mass measured for the 105 optically blue galaxies is (19.0 ± 6.5) × 109 M⊙ . 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 H i gas content than galaxies further away where the galaxy density is lower. The optically blue galaxies contain the majority of the H i gas surrounding the cluster. However, there is evidence that the optically red galaxies contain appreciable quantities of H i gas within their central regions. The Abell 370 galaxies have H i mass-to-optical-light ratios similar to local galaxy samples and have the same correlation between their star formation rate and H i mass as found in the nearby galaxies. The average star formation rate derived from [O ii] 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 H i 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.
Monthly Notices of the Royal Astronomical Society, Vol. 399, no. 3 (Nov 2009), pp. 1447-1470