Gas, Stars and Star Formation in ALFALFA Dwarf Galaxies
  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.
