paper

Gas, Stars and Star Formation in ALFALFA Dwarf Galaxies

arXiv:1203.3226 · doi:10.1088/0004-6256/143/6/133

Abstract

We examine the global properties of the stellar and HI components of 229 low HI mass dwarf galaxies extracted from the ALFALFA survey, including a complete sample of 176 galaxies with HI masses < 10^{7.7} M_sun and HI line widths < 80 km s^{-1}. SDSS data are combined with photometric properties derived from GALEX to derive stellar masses (M_*) and star formation rates (SFRs) by fitting their UV-optical spectral energy distributions (SEDs). In optical images, many of the ALFALFA dwarfs are faint and of low surface brightness; only 56% of those within the SDSS footprint have a counterpart in the SDSS spectroscopic survey. A large fraction of the dwarfs have high specific star formation rates (SSFRs) and estimates of their SFRs and M_* obtained by SED fitting are systematically smaller than ones derived via standard formulae assuming a constant SFR. The increased dispersion of the SSFR distribution at M_* < 10^8 M_sun is driven by a set of dwarf galaxies that have low gas fractions and SSFRs; some of these are dE/dSphs in the Virgo cluster. The imposition of an upper HI mass limit yields the selection of a sample with lower gas fractions for their M_* than found for the overall ALFALFA population. Many of the ALFALFA dwarfs, particularly the Virgo members, have HI depletion timescales shorter than a Hubble time. An examination of the dwarf galaxies within the full ALFALFA population in the context of global star formation laws is consistent with the general assumptions that gas-rich galaxies have lower star formation efficiencies than do optically selected populations and that HI disks are more extended than stellar ones.

34 pages, 15 figures, 1 table; accepted for publication in The Astronomical Journal

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