Studies of the Virgo cluster. III - A classification system and an illustrated atlas of Virgo cluster dwarf galaxies

Abstract

Photographs enlarged to a common scale are given for 138 dwarf galaxies in the region of the Virgo cluster. Most are cluster members, as judged either from their uniquely low surface brightness and/or morphology, or occasionally from velocity data. All known Hubble galaxy types have been found in the Virgo cluster, ranging in absolute magnitude from the brightest known giant ellipticals and spirals to all the types of dwarfs that were expected from prior knowledge of the dE, Sm, Im, and blue compact dwarfs (BCD) in the Local Group and its environs. A new type of very large diameter (10000 pc), low central surface brightness (>25 B mag/arcsec) galaxy, that comes in both early (i.e., dE) and late (i.e., Im V) types, has been isolated, but there are, as yet, no known examples in the local neighborhood. The Atlas is organized in a way that recognizes the continuum between the giant and the dwarf ellipticals on the one hand, and the linear progression which, in order, connects the high luminosity Sc and Sd galaxies, the Magellanic Cloud Sm types, and the SMC Im galaxies of decreasing average surface brightness. This sequence among the late-type galaxies is one of ever-decreasing absolute luminosity. It is emphasized that, unlike the continuum that exists between giant and dwarf E galaxies, there are no connectives from the giant Sa, Sb, and Sc galaxies to a sequence of very-low-luminosity spirak. Dwarf Sa, Sb, and Sc galaxies (i.e., fainter than, say MB = - 17) are not present in the Virgo cluster, and we surmise that they do not exist. This Atlas and its descriptions are meant to form a preliminary classification system that organizes all known types of low-luminosity galaxies into a coherent scheme. It is also meant as a preliminary candidate list of late-type, low-SB dwarfs that can be usefully observed with Space Telescope to resolve the brightest stars in a next step to establish the extragalactic distance scale.