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

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 RF=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/SOS 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.