The EDGE-CALIFA survey: using optical extinction to probe the spatially resolved distribution of gas in nearby galaxies

We present an empirical relation between the cold gas surface density (Sigma(gas)) and the optical extinction (A(V)) in a sample of 103 galaxies from the Extragalactic Database for Galaxy Evolution (EDGE) survey. This survey provides CARMA interferometric CO observations for 126 galaxies included in the Calar Alto Legacy Integral Field Area (CALIFA) survey. The matched, spatially resolved nature of these data sets allows us to derive the Sigma(gas)-A(V) relation on global, radial, and kpc (spaxel) scales. We determine A(V) from the Balmer decrement (H alpha/H beta). We find that the best fit for this relation is Sigma(gas) (M-circle dot pc(-2)) similar to 26 x A(V) (mag), and that it does not depend on the spatial scale used for the fit. However, the scatter in the fits increases as we probe smaller spatial scales, reflecting the complex relative spatial distributions of stars, gas, and dust. We investigate the Sigma(gas)/A(V) ratio on radial and spaxel scales as a function of EW(H alpha). We find that at larger values of EW(H alpha) (i.e. actively star-forming regions) this ratio tends to converge to twice the value expected for a foreground dust screen geometry (similar to 30 M-circle dot pc(-2) mag(-1)). On radial scales, we do not find a significant relation between the Sigma(gas)/A(V) ratio and the ionized gas metallicity. We contrast our estimates of Sigma(gas) using A(V) with compilations in the literature of the gas fraction on global and radial scales as well as with well-known scaling relations such as the radial star formation law and the Sigma(gas)-Sigma(*) relation. These tests show that optical extinction is a reliable proxy for estimating Sigma(gas) in the absence of direct sub/millimeter observations of the cold gas.