Dwarf galaxies in medium redshift clusters

The large numbers of galaxies seen in deep number-magnitude counts, combined with the fairly low redshifts seen in redshift surveys & faint magnitudes, suggest the presence of an evolving population, of dwarf (ie. low mass) galaxies. However, direct determination of the evolution of field galaxies is observationally very difficult. Indeed, even the local luminosity function may be subject to error because of selection biases. The alternative approach to the luminosity function problem is to consider galaxy clusters. We discuss here preliminary results from an observational study of several medium distance rich clusters of galaxies. Since we have clusters at a range of redshifts from ∼ 0.1 to ∼ 0.3 we can study the evolution (or otherwise) of the (intrinsically) faint galaxy population over a significant span in look-back time. At the higher end we are directly exploring the epochs at which most galaxies are seen in the faint redshift surveys. By observing at red wavelengths we may also hope to see a closer reflection of the mass function than would be obtained by observations in blue light (which can be dominated by transient or rapidly evolving star formation activity). Our present results indicate a remarkable constancy of the faint end of the luminosity/mass function for dense, rich clusters over the last quarter of a Hubble time. This may place limits on the allowed efficacy of mergers in the evolution of cluster galaxies. In addition, we have also observed a range of morphological types of cluster in order to check on the possible environmental dependence of the dwarf galaxy population at given redshift.