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Session 84 - Dwarf Galaxies.
Oral session, Wednesday, January 15
The Local Group and nearby environs contain a wide variety of dwarf galaxy types with different star formation rates and histories. This variety creates an opportunity: by studying the properties of the interstellar medium (ISM) in different types of nearby dwarf galaxies, we learn not only about the evolution of the individual galaxies but also about the relation between star formation and the ISM. We present a detailed study of the neutral ISM (atomic and molecular gas) in a small sample of Local Group and almost-Local Group dwarf galaxies. The five galaxies observed include two dwarf ellipticals, NGC 185 and 205; two dwarf irregulars, Leo A and Sag DIG; and one intermediate spheroidal/irregular, LGS 3. The HI component of each galaxy was imaged at spatial resolutions of 60--160 pc. CO observations of the dwarf ellipticals NGC 185 and 205 trace the molecular phase in those galaxies at comparable resolutions to the HI data. Analysis of the HI line profiles in the dwarf galaxies reveals a low dispersion component (\sigma = 3--4 km/s), localized in clumps, in four of the five galaxies. In the dwarf ellipticals, at least one of these low dispersion atomic clumps is clearly associated with molecular gas and dust obscuration. In the dwarf irregulars, the low dispersion HI component is embedded in a diffuse, higher dispersion (\sigma = 8 km/s) HI component, and thus the two components may be the analogs of the Galactic cold and warm neutral phases. LGS 3, the least massive member of the sample and the galaxy with the least amount of current star formation, shows no evidence for the colder, denser HI phase. Comparisons between the different galaxies give some insight into which factors (supernova rate, metallicity, etc.) regulate the state of the ISM, and such comparisons also probe the dependence of the star formation rate on observable properties of the ISM.
Program listing for Wednesday