Optical–infrared studies of the rich cluster of galaxies Abell 370 at z = 0.37

Using the infrared camera IRCAM on the 3.8-m UK Infrared Telescope we have added new $$\rm K(\equiv 2-\mu m)$$ photometry to existing optical CCD measurements for a complete sample of 53 galaxies with K < 17.5 mag in the intermediate-redshift rich cluster Abell 370 (z = 0.37). We demonstrate techniques for processing 2-μm images that achieve flatness to 0.04 per cent and a 1 σ surface-brightness detection limit of $$\mu_{K} =21.5)$$ mag arcsec ^–2. The mean optical–infrared colour–magnitude relation for early-type galaxies expected to be members on the basis of both spectroscopy and optical colours, is in excellent agreement with that deduced locally when redshift corrections are applied. In contrast to earlier results, we find no evidence for significant galactic reddening in this direction. However, a large proportion ( 60 per cent) of red members appear to be ≃ 0.1 mag redder in R–K than present-day ellipticals, and a high proportion of these have spectroscopic features indicative of the post-starburst phase. Following earlier suggestions, we examine a model whereby a significant fraction of early-type members suffer short-term bursts of star formation. The red excess found in the colour–magnitude relation of Abell 370 can be explained in terms of a contribution by asymptotic giant-branch stars to the post-burst phase, but the high fraction observed seems to imply that virtually all cluster galaxies must suffer this activity at some point in their evolution.