ARE LUMINOUS CD HALOS FORMED BY THE DISRUPTION OF DWARF GALAXIES

From a total sample of 45 Abell clusters observed by the Einstein X-ra y observatory, we present the results on the galaxy luminosity functio n (LF) for a group of seven clusters that were identified by the morph ology of their LFs. The LFs were derived using photometric data to a c ompleteness limit similar to 5.5 mag below M. We found that a single Schechter function with an average alpha = -1.0 gives a good fit to th ese individual LFs within the magnitude range. These seven clusters ha ve common properties, which indicate they form a homogeneous class of dynamically evolved clusters that can be characterized by the presence of a dominant cD galaxy, high richness, symmetrical single-peaked X-r ay emission, and high gas mass. On the other hand, steep faint-end slo pes (-2.0 less than or equal to alpha less than or equal to -1.4) are usually detected in poorer clusters. Our result gives a direct indicat ion that the faint-end slope of the galaxy LF is subject to environmen tal effects. We propose that the flatness of the faint-end slope in th ese clusters results from the disruption of a large fraction of dwarf galaxies during the early stages of cluster evolution. The stars and g as from the disrupted galaxies are redistributed throughout the cluste r potential during violent relaxation. This heuristic scenario can exp lain the origin of the luminous halos of cD galaxies and a large fract ion of the gas content in the intracluster medium as a by-product. The correlation between the cluster gas mass determined from the modeling of the X-ray emission and the cD halo optical luminosity is presented to support the proposed model.