The EDGE-CALIFA survey: The resolved star formation efficiency and local
  physical conditions
  We measure the star formation rate (SFR) per unit gas mass and the star
formation efficiency (SFE$_{\rm gas}$ for total gas, SFE$_{\rm mol}$ for the
molecular gas) in 81 nearby galaxies selected from the EDGE-CALIFA survey,
using $^{12}$CO(J=1-0) and optical IFU data. For this analysis we stack CO
spectra coherently by using the velocities of H$\alpha$ detections to detect
fainter CO emission out to galactocentric radii $r_{\rm gal} \sim 1.2 r_{25}$
($\sim 3 R_{\rm e}$), and include the effects of metallicity and high surface
densities in the CO-to-H$_2$ conversion. We determine the scale lengths for the
molecular and stellar components, finding a close to 1:1 relation between them.
This result indicates that CO emission and star formation activity are closely
related. We examine the radial dependence of SFE$_{\rm gas}$ on physical
parameters such as galactocentric radius, stellar surface density
$\Sigma_{\star}$, dynamical equilibrium pressure $P_{\rm DE}$, orbital
timescale $\tau_{\rm orb}$, and the Toomre $Q$ stability parameter (including
star and gas $Q_{\rm star+gas}$). We observe a generally smooth, continuous
exponential decline in the SFE$_{\rm gas}$ with $r_{\rm gal}$. The SFE$_{\rm
gas}$ dependence on most of the physical quantities appears to be well
described by a power-law. Our results also show a flattening in the SFE$_{\rm
gas}$-$\tau_{\rm orb}$ relation at $\log[\tau_{\rm orb}]\sim 7.9-8.1$ and a
morphological dependence of the SFE$_{\rm gas}$ per orbital time, which may
reflect star formation quenching due to the presence of a bulge component. We
do not find a clear correlation between SFE$_{\rm gas}$ and $Q_{\rm star+gas}$.
