THE FAINT END OF THE GALAXY LUMINOSITY FUNCTION IN MODERATE-REDSHIFT CLUSTERS

We present deep two-colour photometry of two rich clusters at z = 0.18 , A665 and A1689. We use these data to construct number counts as a fu nction of magnitude in the two fields. By combining these counts with similar observations from a large-area field survey, we subtract the f ield contamination statistically to produce luminosity functions for t he two clusters. Great care has been taken to achieve agreement betwee n the photometry of these two samples. The cluster data are complete t o a limiting magnitude of I = 22.5 or an absolute magnitude in the clu ster of I = -18.0 (M + 5). The luminosity functions of both clusters are well described by a Gaussian function for the bright galaxies, com bined with a Schechter function at the faint end, similar to that requ ired to fit the luminosity function in local clusters. The slope at th e faint end of the Schechter function in both clusters is extremely st eep in V, alpha similar to -2. A shallower slope is seen to the limit of the I data, indicating that the cluster population is rapidly bluei ng as we reach fainter magnitudes. The excellent agreement between the forms of the luminosity functions in our two distant clusters, as wel l as the agreement with the luminosity function given by Driver et al. for a single z = 0.21 cluster, indicates that this faint blue populat ion is a general constituent of distant clusters. We compare our resul ts with those from studies of local clusters. Depending upon the degre e of fading (or disruption) of these faint blue galaxies, we tentative ly identify their remnants with the low surface brightness dwarf galax ies which are the dominant population in local clusters. We discuss th e possible role of this population as the source of most of the X-ray gas in rich clusters.