The Progenitors of Dwarf Spheroidal Galaxies

Dwarf spheroidal (dSph) galaxies present an evolutionary puzzle that weexplore in 40 early- and late-type dwarfs in the Local Group and nearby field.Although dSphs formed stars over extended periods, today all but one are freeof detectable interstellar matter (ISM), even in the Fornax dSph, where starsstill formed 100 Myr ago. Combining metallicities for red giants with HI datafrom the literature, we show that the well-known offset inluminosity-metallicity (L-Z) relations for dSphs and dwarf irregular (dIrr)galaxies exists also when comparing only their old stellar populations: dSphshave higher mean stellar metallicities for a fixed luminosity. Evidentlyyounger dSphs experienced more efficient enrichment than young dIrrs. Dwarfgalaxies, whose locus in the L-Z diagram is consistent with that of dSphs evenfor baryonic luminosities, are the ``transition-type dwarfs'' Phoenix, DDO210,LGS3, Antlia, and KKR25. They have mixed dIrr/dSph morphologies, low stellarmasses, low angular momentum, and HI contents of less than a few 10^6 solarmasses. Unlike dIrrs, many transition-type dwarfs would closely resemble dSphsif their gas were removed; they are likely dSph progenitors. As gas removal iskey, we consider the empirical evidence for various gas removal processes. Wesuggest that internal gas removal mechanisms are inadequate and favor rampressure stripping to make dSphs. A combination of initial conditions andenvironment seems to support the formation of dSphs, which appear to form fromsmall galaxies with active early star formation, whose evolution halts due toexternally induced gas loss. Transition-type dwarfs then are dSphs that kepttheir ISM, and therefore should replace dSphs in isolated locations wherestripping is ineffective. (Abridged)Comment: 25 pages in AASTeX two-column preprint style, 1 table, 3 figures. Accepted for publication in the Astronomical Journal (April 2003 issue