Hubble Space Telescope survey of the Perseus Cluster - I. The structure and dark matter content of cluster dwarf spheroidals

We present the results of a Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) study of dwarf galaxies in the core of the rich nearby Perseus Cluster, down to M_V = -12. We identify 29 dwarfs as cluster members, 17 of which are previously unstudied. All the dwarfs we examine are remarkably smooth in appearance, and lack internal features. This smoothness is quantified by parametric fitting of the two-dimensional light distribution, and through non-parametric structural parameters. The dwarfs in this study are found to be very smooth, and have very low asymmetry and clumpiness values (<A> = 0.03 +/- 0.04,<S> = 0.02 +/- 0.09) throughout their entire structure, showing no evidence for internal features or star formation that could be the result of tidal processes or star formation induced by the cluster environment. Based on these observations, and the sizes of these dwarfs, we argue that some of the dwarfs in our sample must have a large dark matter content to prevent disruption by the cluster potential. We derive a new method, independent of kinematics, for measuring the dark matter content of dEs, based on the radius of the dwarf, the projected distance of the dwarf from the cluster centre, and the total mass of the cluster interior to it. We use this method to determine the mass-to-light ratios for 25 of the dwarfs in our sample, the others being too close to bright stars to accurately determine their radii. At their current positions in the cluster, 12 out of 25 of the dwarfs in our sample require dark matter to remain stable against the cluster potential. These 12 dwarfs are all fainter than M_V = -14, and nearly all dEs with M_V < -13 require dark matter to survive. We also consider the mass that the dwarfs would require to survive the tidal forces at the centre of the cluster, at a pericentric distance of 35 kpc from the cluster centre. We find that at this distance, the mass-to-light ratios of these dwarfs are comparable to those of the Local Group dSphs, ranging between M_solar/L_solar ~ 1 and 120.