The EDGE-CALIFA survey: The resolved star formation efficiency and local physical conditions

Abstract

We measure the star formation rate (SFR) per unit gas mass and the star formation efficiency (SFE${}_{\mathrm{gas}}$ for total gas, SFE${}_{\mathrm{mol}}$ 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 $r_{\mathrm{gal}}\sim 1.2r_{25}$ ($\sim 3R_{\mathrm{e}}$), 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${}_{\mathrm{gas}}$ on physical parameters such as galactocentric radius, stellar surface density ${\Sigma }_{\star }$, dynamical equilibrium pressure $P_{\mathrm{DE}}$, orbital timescale ${\tau }_{\mathrm{orb}}$, and the Toomre $Q$ stability parameter (including star and gas $Q_{\mathrm{star}+\mathrm{gas}}$). We observe a generally smooth, continuous exponential decline in the SFE${}_{\mathrm{gas}}$ with $r_{\mathrm{gal}}$. The SFE${}_{\mathrm{gas}}$ 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${}_{\mathrm{gas}}$-${\tau }_{\mathrm{orb}}$ relation at $\log [{\tau }_{\mathrm{orb}}]\sim 7.9-8.1$ and a morphological dependence of the SFE${}_{\mathrm{gas}}$ 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${}_{\mathrm{gas}}$ and $Q_{\mathrm{star}+\mathrm{gas}}$.

24 pages, 12 figures, 1 Table

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