THE SURVEY OF H I IN EXTREMELY LOW-MASS DWARFS (SHIELD)

Abstract

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