The Galaxy Luminosity Function in Clusters and the Field

We present the results from a CCD survey of the B-band luminosity functions of nine clusters of galaxies, and compare them to published photographic luminosity functions of nearby poor clusters like Virgo and Fornax and also to the field luminosity function. We derive a composite luminosity function by taking the weighted mean of all the individual cluster luminosity functions; this composite luminosity function is steep at bright and faint magnitudes and is shallow in-between. All clusters have luminosity functions consistent with this single composite function. This is true both for rich clusters like Coma and for poor clusters like Virgo. This same composite function is also individually consistent with the deep field lumi-nosity functions of Cowie et al. (1996) and Ellis et al. (1996), and also with the faint-end of the Las Campanas Redshift Survey R-band luminosity function, shifted by 1.5 magnitudes. A comparison with the Loveday et al. (1992) field luminosity function which is well-determined at the bright-end, shows that the composite function that fits the field data well fainter than M B = −19 drops too steeply between M B = −19 and M B = −22 to fit the field data there well. 1 INTRODUCTION The galaxy luminosity function φ(L), defined as the number density of galaxies per unit luminosity, is one of the most direct observational probes of galaxy formation theories. Most evidence (see references below) suggests that φ(L) drops steeply brighter than about M B = −21 (i.e., extremely luminous galaxies are very rare), and rises more gradually fainter than this. This form is expressed well by the Schechter (1976) function. At very faint magnitudes M B > −10, we expect on theoretical grounds that φ(L) turns over again, as we expect star formation in very low mass systems to be suppressed by photoionization of the intergalactic medium from the ultraviolet background but this turnover has not yet been observed. It is also well established that different types of galaxies contribute proportionately different amounts to φ(L) at different magnitudes. At bright magnitudes M B > −16, most galaxies in clusters are giant ellipticals and in the field they are giant late-type galaxies. At fainter magnitudes M B < −16, most galaxies in clusters are dwarf spheroidal galaxies (see Binggeli 1987 − note that he calls them dwarf ellipticals), and in the field, they are either dwarf spheroidal or dwarf spiral and irregular galaxies. The different …