ESO540-032: a Transition-Type Dwarf in the Sculptor Group

Abstract

The dwarf Irregular galaxies (dIrr) are gas-rich systems, with hydrogen mass to blue luminosity ratio (MHI/LB) values exceeding one in solar units, and active star formation. The dwarf Elliptical galaxies (dE), on the other hand, are usually gas-free, with (MHI/LB) values substantially less than one. In most instances they are not actively forming stars, although their star formation histories are complex and varied. These dwarf galaxy types exhibit a morphology-density relation, in the sense that dE galaxies tend to be found in denser environments. For example, within the Local Group, the vast majority of the dE systems are found as companions to the Milky Way or M31, while the dIrr members are mostly relatively isolated. In the Local Group there are also dwarf galaxies that are transition-types, systems that possess moderate amounts of gas (MHI/LB ≈ 0.1–0.5) and a lowlevel of current or recent star formation within a dominant older population. We note in passing that the recently discovered distant Milky Way companion Leo T [5] is more likely a low luminosity dwarf irregular, than a genuine transition-type galaxy, as it appears MHI/LB for this dwarf exceeds one [5]. The relationship between these classes of dwarf galaxies remains controversial: e.g., [4] argue that the existence of an offset in the luminosity-metallicity relation between dEs and dIrrs indicates different evolutionary paths. They suggest that transition-type dwarfs are the progenitors of dE (dSph) systems, in the sense that in low density environments where ram-pressure stripping mechanisms are ineffective, transition-type dwarfs should be common [4]. One obvious way to test this suggestion is to investigate the properties of dwarf galaxies in environments beyond the Local Group. The Sculptor Group is a low density aggregation of galaxies ranging in distance from ∼1.5 to ∼4 Mpc. It contains at least five low-luminosity early-type dwarf galaxies. Their neutral gas content has been studied by [1], who found that four are likely