SHIELD: COMPARING GAS AND STAR FORMATION IN LOW-MASS GALAXIES

Abstract

ABSTRACT We analyze the relationships between atomic, neutral hydrogen (H i ) and star formation (SF) in the 12 low-mass SHIELD galaxies. We compare high spectral (∼0.82 km s −1 ch −1 ) and spatial resolution (physical resolutions of 160–640 pc) H i imaging from the VLA with H α and far-ultraviolet imaging. We quantify the degree of co-spatiality between star-forming regions and regions of high H i column densities. We calculate the global star formation efficiencies (SFE; <?CDATA ${{\rm{\Sigma }}}_{\mathrm{SFR}}$?> / <?CDATA ${{\rm{\Sigma }}}_{{\rm{H}}{\rm{I}}}$?> ) and examine the relationships among the SFE and H i mass, H i column density, and star formation rate (SFR). The systems are consuming their cold neutral gas on timescales of order a few gigayears. While we derive an index for the Kennicutt–Schmidt relation of N ≈ 0.68 ± 0.04 for the SHIELD sample as a whole, the values of N vary considerably from system to system. By supplementing SHIELD results with those from other surveys, we find that H i mass and UV-based SFR are strongly correlated over five orders of magnitude. Identification of patterns within the SHIELD sample allows us to bin the galaxies into three general categories: (1) mainly co-spatial H i and SF regions, found in systems with the highest peak H i column densities and highest total H i masses; (2) moderately correlated H i and SF regions, found in systems with moderate H i column densities; and (3) obvious offsets between H i and SF peaks, found in systems with the lowest total H i masses. SF in these galaxies is dominated by stochasticity and random fluctuations in their ISM.