AAS 201st Meeting, January, 2003
Session 116. Galaxies - Activating
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[116.13] The Progenitors of Dwarf Spheroidal Galaxies

E. K. Grebel (MPIA), J. S. Gallagher (U Wisc.), D. Harbeck (MPIA)

The gas-deficient dwarf spheroidal (dSph) galaxies are an evolutionary puzzle that we explore in early-type and late-type dwarfs in the Local Group and in the nearby field. Consistent stellar metallicity estimates from red giant branches are derived and are combined with high sensitivity H\,{\sc i} 21-cm line data from the literature. Although dSphs experienced star formation over extended time spans in their youths, today all but one are completely free of detectable interstellar material, even in cases like Fornax, where stars formed in the last hundred million years. We confirm the well-established offset in luminosity-metallicity relationships for dSphs and dwarf irregular (dIrr) galaxies: dSphs have higher mean stellar metallicities for a fixed optical luminosity. Galaxies whose older stellar populations resemble those of dSphs are the ``transition-type dwarfs,'' which have mixed dIrr/dSph morphologies, low stellar masses, and H\,{\sc i} contents of at most a few 106 M\odot. Unlike dIrrs, transition-type dwarfs would closely resemble dSphs if their gas were removed, as required to become a dSph; they are likely dSph progenitors. Among possible gas removal processes, internal mechanisms are found to be inadequate; ram pressure stripping by gas in galaxy halos and an inter-group medium is favored to clean the bulk of interstellar matter from dSphs. Thus dSph galaxies may arise from a combination of rapid early evolution and effective gas removal. True field dSphs therefore should be very rare, but their dSph/dIrr equivalents may be relatively common.

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