Characterizing the Star Formation of the Low-Mass SHIELD Galaxies from
  Hubble Space Telescope Imaging
  The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) is an on-going
multi-wavelength program to characterize the gas, star formation, and evolution
in gas-rich, very low-mass galaxies that populate the faint end of the galaxy
luminosity function. The galaxies were selected from the first ~10% of the HI
ALFALFA survey based on their low HI mass and low baryonic mass. Here, we
measure the star-formation properties from optically resolved stellar
populations for 12 galaxies using a color-magnitude diagram fitting technique.
We derive lifetime average star-formation rates (SFRs), recent SFRs, stellar
masses, and gas fractions. Overall, the recent SFRs are comparable to the
lifetime SFRs with mean birthrate parameter of 1.4, with a surprisingly narrow
standard deviation of 0.7. Two galaxies are classified as dwarf transition
galaxies (dTrans). These dTrans systems have star-formation and gas properties
consistent with the rest of the sample, in agreement with previous results that
some dTrans galaxies may simply be low-luminosity dIrrs. We do not find a
correlation between the recent star-formation activity and the distance to the
nearest neighboring galaxy, suggesting that the star-formation process is not
driven by gravitational interactions, but regulated internally. Further, we
find a broadening in the star-formation and gas properties (i.e., specific
SFRs, stellar masses, and gas fractions) compared to the generally tight
correlation found in more massive galaxies. Overall, the star-formation and gas
properties indicate these very low-mass galaxies host a fluctuating,
non-deterministic, and inefficient star-formation process.
