Dwarf Galaxies in Clusters as Probes of Galaxy Formation and Dark Matter

Abstract

We present the results of a Hubble Space Telescope (HST) ACS and WFPC2 study of dwarf galaxies in the nearby Perseus Cluster, down to M_V = -12, spanning the core and outer regions of this cluster. We examine how properties such as the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. The low masses of dwarf galaxies allow us to determine their environmentally driven based galaxy evolution, the effects of which are harder to examine in massive galaxies. The structures of our dwarfs in both the core and outer regions of the cluster are quantified using the concentration, asymmetry and clumpiness (CAS) parameters. We find that, on average, dwarfs in the outer regions of Perseus are more disturbed than those in the cluster core, with higher asymmetries and clumpier light distributions. We measure the (V-I)_0 colours of the dEs, and find that dwarfs in both the inner and outer regions of the cluster lie on the same colour magnitude relation. Based on these results, we infer that the disturbed dwarfs in the cluster outskirts are likely "transition dwarfs", with their colours transforming before their structures. Finally, we infer from the smoothness of the cluster core population that dwarfs in the inner regions of the cluster must be highly dark matter dominated to prevent their disruption by the cluster potential. We derive a new method to determine the minimum mass the dwarfs must have to prevent this disruption without the need for resolved spectroscopy, and determine their mass-to-light ratios. At their orbit pericentre, dwarfs in the core of Perseus require mass-to-light ratios between 1 and 120 to prevent their disruption, comparable to those found for the Local Group dSphs.