POPULATION STUDIES IN GROUPS AND CLUSTERS OF GALAXIES .4. COMPARISON OF THE LUMINOSITY FUNCTIONS AND MORPHOLOGICAL-TYPE DISTRIBUTIONS IN 7 NEARBY GROUPS

Luminosity functions and morphological type fractions are examined in seven nearby groups of galaxies (Leo, Dorado, NGC 1400, NGC 5044, Antlia, Fornax, and Virgo). The groups are found to be similar in their fractions of early and late-type galaxies, consistent with the Dressler morphology-density relation and their similar mean densities. However, the dwarf-to-giant ratio for early type galaxies varies significantly with richness. The Virgo Cluster has approximately 5 times as many dwarfs per giant as the poorest groups (Leo, Dorado, and NGC 1400). In addition, the early type dwarf-to-giant ratio increases monotonically with the richness of the groups, and is possibly correlated with velocity dispersion. There is no evidence for radial gradients in the dwarf-to-giant ratio in the Antlia, Fornax, and Virgo Clusters. The composite giant + dwarf luminosity functions are not well fit by a Schechter function, but are best approximated by a bounded function (e.g., a Gaussian) for the giants, and a Schechter function with a faint end slope alpha approximately -1.3 for the dwarfs. The difference between the flat slopes alpha approximately -1 generally found for field and group samples, and the steeper alpha approximately -1.25 generally found for clusters is reproduced in our sample when we restrict the data to galaxies brighter than M(BT) = -16. However, the very faint end -12.5 greater-than-or-equal-to M(BT) greater-than-or-equal-to -16 slope of the luminosity function in the poor groups is at least as steep as that in the Virgo Cluster. The conclusions are (1) although the shapes of the individual dwarf and giant luminosity functions remain the same, the ratio of the numbers of dwarfs to giants changes with environment; (2) this difference in the mixing ratio for the total luminosity function produces an artifical flattening at the faint end of the total function in those environments where the dwarf-to-giant ratio is low, such as in the field; and (3) it is likely that the faint end slope of the dwarf luminosity function is identical at alpha approximately -1.3 in all environments.