Distance Determinations to SHIELD Galaxies from HST Imaging
  The Survey of HI in Extremely Low-mass Dwarf galaxies (SHIELD) is an on-going
multi-wavelength program to characterize the gas, star formation, and evolution
in gas-rich, very low-mass galaxies. The galaxies were selected from the first
~10% of the HI ALFALFA survey based on their inferred low HI mass and low
baryonic mass, and all systems have recent star formation. Thus, the SHIELD
sample probes the faint end of the galaxy luminosity function for star-forming
galaxies. Here, we measure the distances to the 12 SHIELD galaxies to be
between 5-12 Mpc by applying the tip of the red giant method to the resolved
stellar populations imaged by the Hubble Space Telescope. Based on these
distances, the HI masses in the sample range from $4\times10^6$ to
$6\times10^7$ M$_{\odot}$, with a median HI mass of $1\times10^7$ M$_{\odot}$.
The TRGB distances are up to 73% farther than flow-model estimates in the
ALFALFA catalog. Because of the relatively large uncertainties of flow model
distances, we are biased towards selecting galaxies from the ALFALFA catalog
where the flow model underestimates the true distances. The measured distances
allow for an assessment of the native environments around the sample members.
Five of the galaxies are part of the NGC 672 and NGC 784 groups, which together
constitute a single structure. One galaxy is part of a larger linear ensemble
of 9 systems that stretches 1.6 Mpc from end to end. Two galaxies reside in
regions with 1-4 neighbors, and four galaxies are truly isolated with no known
system identified within a radius of 1 Mpc.
