The Survey of HI in Extremely Low-mass Dwarfs (SHIELD)
  We present first results from the "Survey of HI in Extremely Low-mass Dwarfs"
(SHIELD), a multi-configuration EVLA study of the neutral gas contents and
dynamics of galaxies with HI masses in the 10^6-10^7 Solar mass range detected
by the Arecibo Legacy Fast ALFA (ALFALFA) survey. We describe the survey
motivation and concept demonstration using VLA imaging of 6 low-mass galaxies
detected in early ALFALFA data products. We then describe the primary
scientific goals of SHIELD and present preliminary EVLA and WIYN 3.5m imaging
of the 12 SHIELD galaxies. With only a few exceptions, the neutral gas
distributions of these extremely low-mass galaxies are centrally concentrated.
In only 1 system have we detected HI column densities higher than 10^21 cm^-2.
Despite this, the stellar populations of all of these systems are dominated by
blue stars. Further, we find ongoing star formation as traced by H alpha
emission in 10 of the 11 galaxies with H alpha imaging obtained to date. Taken
together these results suggest that extremely low-mass galaxies are forming
stars in conditions different from those found in more massive systems. While
detailed dynamical analysis requires the completion of data acquisition, the
most well-resolved system is amenable to meaningful position-velocity analysis.
For AGC 749237, we find well-ordered rotation of 30 km/s at a distance of ~40
arcseconds from the dynamical center. At the adopted distance of 3.2 Mpc, this
implies the presence of a >1x10^8 Solar mass dark matter halo and a baryon
fraction < ~0.1.
