The Progenitors of Dwarf Spheroidal Galaxies

Abstract

The gas-deficient dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that we explore in 40 early-type and late-type dwarfs in the Local Group and nearby field. Although dSph's experienced star formation over extended time spans in their youths, today all but one are completely free of detectable interstellar material, even in the Fornax dSph, where stars formed in the last 100 Myr. Combining photometric and spectroscopic stellar metallicity estimates for red giant branches with high-sensitivity H I 21 cm line data from the literature, we show that the well-established offset in luminosity-metallicity relationships for dSph's and dwarf irregular (dIrr) galaxies exists also when confining the comparison to their old stellar populations: dSph's have higher mean stellar metallicities for a fixed optical luminosity. Evidently star formation in younger dSph's was more vigorous than in the youthful dIrr's, leading to more efficient enrichment. Dwarf galaxies, whose locus in the luminosity-metallicity diagram is consistent with that of dSph's, even when baryonic luminosities are considered, are the "transition-type dwarfs" Phoenix, DDO 210, LGS 3, Antlia, and KKR 25. These dwarfs have mixed dIrr/dSph morphologies, low stellar masses, low angular momentum, and H I contents of at most a few 10 6 M ⊙ . Unlike dIrr's many transition-type dwarfs would closely resemble dSph's if their gas were removed, as required to become a dSph; they are likely dSph progenitors. As gas removal is the key factor for such a transition, we consider the empirical evidence in favor and against various gas removal processes. We suggest that internal gas removal mechanisms are inadequate and favor ram-pressure stripping to clean the bulk of interstellar matter from galaxies to make dSph's. A combination of initial conditions and environment seems to support the formation of dSph's: nearby dSph's appear to form from small galaxies with active early star formation, whose evolution halts due to externally induced gas loss. Transition-type dwarfs, then, are dSph's that kept their interstellar medium and therefore should replace dSph's in isolated locations where stripping is ineffective.