Cloud-scale ISM Structure and Star Formation in M51

Abstract

We compare the structure of molecular gas at 40pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370pc and 1.1kpc resolution elements, and within each we estimate the molecular gas depletion time (tDep mol ), the star-formation efficiency per free-fall time ( ff ), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Σ, line width, σ, and b º S s2 μ avir 1, a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star-formation efficiency per free-fall time,  ff (áS40 pcñ) ~ 0.3%–0.36%, lower than adopted in many models and found for local clouds. Furthermore, the efficiency per free-fall time anti-correlates with both Σ and σ, in some tension with turbulent star-formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is b º S s2, tracing the strength of self-gravity, with tDep μ bmol 0.9. The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low tDep mol ). The different regions of the galaxy mostly overlap in tDep mol as a function of b, so that low b explains the surprisingly high tDep mol found toward the inner spiral arms found by Meidt et al. (2013).