Elise Nims, John M. Cannon, Ian Cave, Cedric Hagen, Melissa V. Marshall, Clara M. Thomann, Yaron G. Teich, Andrew T. McNichols, E. C. Elson, Nathalie Haurberg, Gyula I. G. Józsa, Jürgen Ott, Amelie Saintonge, Steven R. Warren, Shan Huang, Angela Van Sistine, Elizabeth A. K. Adams, Riccardo Giovanelli, Martha P. Haynes, and Evan D. Skillman
ATOMIC hydrogen, STELLAR evolution, DWARF galaxies, LOW mass stars, and SPECTRAL imaging
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; / ) 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. [ABSTRACT FROM AUTHOR]
Steven R. Warren, Edward Molter, John M. Cannon, Alberto D. Bolatto, Elizabeth A. K. Adams, Elijah Z. Bernstein-Cooper, Riccardo Giovanelli, Martha P. Haynes, Rodrigo Herrera-Camus, Katie Jameson, Kristen B. W. McQuinn, Katherine L. Rhode, John J. Salzer, and Evan D. Skillman
DWARF galaxies, MAGELLANIC clouds, ASTRONOMICAL surveys, COSMOLOGY, and INTERFEROMETERS
We present sensitive CO (J = 1 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z ∼ 3% Z⊙), Sextans A (Z ∼ 7.5% Z⊙), and Sextans B (Z ∼ 7.5% Z⊙), all obtained with the Combined Array for Millimeter-wave Astronomy interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4σ) upper limits on the CO luminosity (LCO) in these metal-poor galaxies. We find the CO luminosities to be LCO < 2900 K km s−1 pc2 for Leo P, LCO < 12,400 K km s−1 pc2 for Sextans A, and LCO < 9700 K km s−1 pc2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between LCO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen. [ABSTRACT FROM AUTHOR]
Bernstein-Cooper, Elijah Z., Cannon, John M., Elson, Edward C., Warren, Steven R., Chengular, Jayaram, Skillman, Evan D., Adams, Elizabeth A. K., Bolatto, Alberto D., Giovanelli, Riccardo, Haynes, Martha P., McQuinn, Kristen B. W., Pardy, Stephen A., Rhode, Katherine L., and Salzer, John J.