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 ($\tau_{ \rm Dep}^{ \rm mol}$), the star formation efficiency per free fall time ($\epsilon_{ \rm ff}$), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, $\Sigma$, line width, $\sigma$, and $b\equiv\Sigma/\sigma^2\propto\alpha_{ \rm vir}^{ -1}$, 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, $\epsilon_{ ff} \left( \left< \Sigma_{ 40pc} \right> \right) \sim 0.3{ -}0.36\%$, lower than adopted in many models and found for local clouds. More, the efficiency per free fall time anti-correlates with both $\Sigma$ and $\sigma$, 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 $b \equiv \Sigma / \sigma^2$, tracing the strength of self gravity, with $\tau_{ \rm Dep}^{ \rm mol} \propto b^{ -0.9}$. The sense of the correlation is that gas with stronger self-gravity (higher $b$) forms stars at a higher rate (low $\tau_{ \rm Dep}^{ \rm mol}$). The different regions of the galaxy mostly overlap in $\tau_{ \rm Dep}^{ \rm mol}$ as a function of $b$, so that low $b$ explains the surprisingly high $\tau_{ \rm Dep}^{ \rm mol}$ found towards the inner spiral arms found by by Meidt et al. (2013).
Categories: astro-ph.GA