Hubble Space Telescope survey of the Perseus cluster - II. Photometric scaling relations in different environments

We investigate the global photometric scaling relations traced by early-type galaxies in different environments, ranging from dwarf spheroidals, over dwarf elliptical galaxies (dEs), up to giant ellipticals (-8mag >~ M_V >~ -24 mag). These results are based, in part, on our new Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) F555W and F814W imagery of dwarf spheroidal galaxies in the Perseus cluster. The full sample, built from our HST images and from data taken from the literature, comprises galaxies residing in the Local Group; the Perseus, Antlia, Virgo and Fornax clusters; and the NGC 5898 and NGC 5504 groups.

Photometric parameters, such as the half-light radius, the central surface brightness and the Sérsic exponent n, are used to parametrize the light distributions and sizes of early-type galaxies. All these parameters vary in a continuous fashion with galaxy luminosity over a range of more than six orders of magnitude in luminosity. We also find that all early-type galaxies follow a single colour-magnitude relation (CMR), which we interpret as a luminosity-metallicity relation for old stellar populations. These scaling relations are almost independent of environment, with Local Group and cluster galaxies coinciding in the various diagrams. As an example, due to the presence of a population of very low surface brightness dwarf spheroidal galaxies (dSphs) in the Fornax cluster, which may be tidally heated dwarf galaxies, the Fornax dwarf spheroidal galaxy (dSph) population is on average only 0.2 mag arcsec^-2 fainter than the Local Group dSph populations. This offset is much too small to destroy the global relation between luminosity and central surface brightness.

We show that at M_V ~ -14mag, the slopes of the photometric scaling relations involving the Sérsic parameters change significantly. This contradicts previous claims that the relations involving Sérsic parameters are pure power laws for all early-type galaxies and are, therefore, more fundamental than other photometric scaling relations derived from them. We argue that these changes in slope reflect the different physical processes that dominate the evolution of early-type galaxies in different mass regimes. As such, these scaling relations contain a wealth of information that can be used to test models for the formation of early-type galaxies.