Cloud Scale ISM Structure and Star Formation in M51
  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).
