Spectral energy distributions of galaxies in high-redshift clusters. III. Abell 370 at z=0.37.

Abstract

We have imaged the distant cluster Abell 370 (z=0.37) with a CCD- based intermediate pass-band multicolour system defined in earlier papers in this series. Our technique is to use such colours to classify faint galaxies in redshift and spectral class without recourse to spectroscopic data. We demonstrate from a new comparison of ~80 spectra gathered in five clusters at high redshift (including Abell 370), that our photometric classifications are generally accurate to +/-0.05 in redshift and to +/-1 Hubble type. For Abell 370 we present classifications for a sample of over 100 galaxies, mostly cluster members, statistically complete to R_F_=21.2. In addition we analyse a deep ultraviolet CCD image of the cluster; this image probes to ~250 nm in the cluster restframe. We find a significant fraction of the optically red galaxies, expected to be E/S0s from their position on the colour-luminosity sequence, show a flux enhancement of 0.5-2 magnitudes at 250 nm, an effect similar to that found in the cluster 0016+16 discussed in Paper II. We also confirm a significant excess of blue probable members with spectral characteristics similar to Scd spirals. The coexistence in a single cluster of the high blue fraction, the red galaxies with ultraviolet excess and galaxies with intermediate colours and Balmer absorption features found by spectroscopic studies, suggests a single evolutionary track whereby galaxies undergo a burst of star formation decaying via the post-starburst and ultraviolet phase to normal E/S0s. The frequency of such behaviour supports the contention that we are witnessing the formation of S0s rather than an erratic evolutionary behaviour associated with normal ellipticals. However, the marked difference between the populations in 0016+16 and Abell 370 suggests such galaxy evolution occurs on locally determined time-scales.