The Survey of HI in Extremely Low-mass Dwarfs (SHIELD)

TL;DR

The SHIELD survey investigates the neutral gas and star formation in extremely low-mass dwarf galaxies, revealing centrally concentrated HI, ongoing star formation despite low gas densities, and evidence of dark matter halos.

Contribution

This study provides the first detailed multi-configuration EVLA imaging of low-mass dwarfs, highlighting their gas distribution, star formation activity, and dynamical properties, which differ from more massive galaxies.

Findings

Most low-mass dwarfs have centrally concentrated HI gas.

Star formation occurs despite low HI column densities.

Detection of ordered rotation suggests significant dark matter halos.

Abstract

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.