S. Phillipps et al., THE LUMINOSITY DISTRIBUTION IN GALAXY CLUSTERS - A DWARF POPULATION-DENSITY RELATION, The Astrophysical journal, 498(2), 1998, pp. 119-123
Recent work suggests that rich clusters of galaxies commonly have larg
e populations of dwarf (i.e., low-luminosity) members, that is, their
luminosity function turns up to a steep slope at the faint end. This p
opulation, or more particularly the relative numbers of dwarfs to gian
ts, appears to be very similar for clusters of similar morphology but
may vary between cluster types. We have previously suggested that dwar
fs may be more common in less compact, spiral-rich clusters. Similarly
, we have found evidence for population gradients across clusters, in
that the dwarf population appears more spatially extended. In the pres
ent Letter, we summarize the current evidence and propose, in an analo
gy to the well-known morphology-density relation, that what we are see
ing is a dwarf population-density relation: dwarfs are more common in
lower density environments. Finally, we discuss recent semianalytic mo
dels of galaxy formation in the hierarchical clustering picture, which
may give clues as to the origin of our proposed relation.