Dwarf Galaxies in Clusters: the Effects of a Violent Environment

New observations suggest that dwarf galaxies pervade the universe, for they have been encountered in large numbers in the field (Marzke et al. 1998), groups (Côté et al. 1977), and clusters of galaxies (e.g. Trentham 1998). Some early studies encountered an over-abundance of dwarfs in clusters (e.g. Sandage et al. 1985) in the nearby universe. Studies at higher redshifts also suggested large numbers of dwarfs in clusters. The faint-end slope of the luminosity function (LF) has indicated the overabundance of dwarf galaxies, having steep values of −2.0 ≤ α ≤ −1.4. Even a pattern of universality in the steep slope has been suggested (Trentham 1998). In the recent studies of clusters’ LFs, a factor that has been often overlooked is the effect of the environment. For instance, the characteristics of cD galaxies have always suggested that their formation is due to processes pertaining to clusters, since no cD galaxy have been found in the field (Kormendy & Djorgovsky 1989). In general cD clusters (Root-Sastry cD; Boutz-Morgan class I, I-II) are rich, massive, and dynamically evolved. In contrast non-cD clusters are often irregular, poorer, and with less hot gas in the intracluster medium (ICM). These cluster morphological and dynamical properties defined mainly by the giant galaxies should also be reflected in the dwarf population. López-Cruz & Yee (1995) encountered a class of clusters that they termed flat-LF clusters that challenged the idea that dwarf galaxies were very abundant in clusters’ environments. Later López-Cruz et al. (1997) reported seven flat-LF clusters in a survey of 45 and suggested that the luminous halos of cD galaxies and a large fraction of the intracluster gas was formed by the disruption of dwarf galaxies. The results of López-Cruz et al. pointed out that the effects of the environment were detectable in the dwarf population easier than in the giant population. The idea that the cluster environment is hostile to galaxies is strengthened by new high resolution numerical simulations showing that the cluster environment is able to strip gas and the outer halos of giant galaxies, even induce morphological changes (e.g., Moore et al. 1999). Dark matter dominated dwarfs should be susceptible to all the dynamical effects induced by the environment. In some cases the effects could be devastating, causing the disruption of a large number of dwarf galaxies, and hence the flat faint-end slopes. A plausible mech-