SHIELD: Comparing Gas and Star Formation in Low-mass Galaxies
McQuinn, Kristen B. W.; Cannon, John M.; Skillman, Evan D.; Salzer, John J.; Haynes, Martha P.; Cave, Ian; Giovanelli, Riccardo; Teich, Yaron G.; McNichols, Andrew T.; Nims, Elise; Adams, Elizabeth A. K.; Bernstein-Cooper, Elijah Z.; Dolphin, Andrew; Elson, E. C.; Haurberg, Nathalie; Józsa, Gyula I. G.; Ott, Jürgen; Saintonge, Amelie; Warren, Steven R.; Hagen, Cedric; Huang, Shan; Janowiecki, Steven; Marshall, Melissa V.; Thomann, Clara M.; Van Sistine, Angela
United States, Netherlands, South Africa, Germany, United Kingdom, Australia
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 Hα 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; {{{Σ }}}{SFR} / {{{Σ }}}{{H}{{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.