Galaxy Properties at the Faint End of the HI Mass Function
  The Survey of HI in Extremely Low-mass Dwarfs (SHIELD) includes a
volumetrically complete sample of 82 gas-rich dwarfs with M_HI~<10^7.2 Msun
selected from the ALFALFA survey. We are obtaining extensive follow-up
observations of the SHIELD galaxies to study their gas, stellar, and chemical
content, and to better understand galaxy evolution at the faint end of the HI
mass function. Here, we investigate the properties of 30 SHIELD galaxies using
Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis
Radio Telescope observations of their neutral hydrogen. We measure tip of the
red giant branch (TRGB) distances, star formation activity, and gas properties.
The TRGB distances are up to 4x greater than estimates from flow models,
highlighting the importance of velocity-independent distance indicators in the
nearby universe. The SHIELD galaxies are in under-dense regions, with 23%
located in voids; one galaxy appears paired with a more massive dwarf. We
quantify galaxy properties at low masses including stellar and HI masses, SFRs,
sSFRs, SFEs, birthrate parameters, and gas fractions. The lowest mass systems
lie below the mass thresholds where stellar mass assembly is predicted to be
impacted by reionization. Even so, we find the star formation properties follow
the same trends as higher mass gas-rich systems, albeit with a different
normalization. The HI disks are small (<r><0.7 kpc) making it difficult to
measure the HI rotation using standard techniques; we develop a new methodology
and report the velocity extent, and its associated spatial extent, with robust
uncertainties.
