Gas and Star Formation in Nearby Galaxies
I will describe what we have learned about the relationship between star formation and gas in nearby galaxies from the combination of new ISM surveys (IRAM HERACLES, VLA THINGS) with shorter-wavelength literature data (SINGS, LVL, the GALEX NGS, and optical data). I will discuss the different relations among HI, H2, and star formation in galaxies, highlighting a new method to extract very sensitive CO measurements using HI priors. I will then discuss scalings between CO and star formation tracers in detail, highlighting breakdowns for low-metallicity galaxies, starbursts, and at cloud scales. Finally, I will show how variations in the H2-to-HI ratio −which has been previously identified as a star formation threshold− depend on environmental factors like ISM density, dust abundance, and gravitational instability.