A new estimator of resolved molecular gas in nearby galaxies

A relationship between dust-reprocessed light from recent star formation and the amount of star-forming gas in a galaxy produces a correlation between Wide-field Infrared Survey Explorer (WISE) 12 mu m emission and CO line emission. Here, we explore this correlation on kiloparsec scales with CO(1-0) maps from EDGE-CALIFA matched in resolution to WISE 12 mu m images. We find strong CO-12 mu m correlations within each galaxy and we show that the scatter in the global CO-12 mu m correlation is largely driven by differences from galaxy to galaxy. The correlation is stronger than that between star formation rate and H-2 surface densities [Sigma(H-2)]. We explore multivariable regression to predict Sigma(H-2) in star-forming pixels using the WISE 12 mu m data combined with global and resolved galaxy properties, and provide the fit parameters for the best estimators. We find that Sigma (H-2) estimators that include Sigma(12 mu m) are able to predict Sigma(H-2) more accurately than estimators that include resolved optical properties instead of Sigma(12 mu m). These results suggest that 12 mu m emission and H2 as traced by CO emission are physically connected at kiloparsec scales. This may be due to a connection between polycyclic aromatic hydrocarbon emission and the presence of H-2. The best single-property estimator is log Sigma(H-2)/M-circle dot p(c-2) = (0.48 +/- 0.01) + (0.71 +/- 0.01) log Sigma(12 mu m)/L-circle dot pc(-2). This correlation can be used to efficiently estimate Sigma(H-2) down to at least 1M(circle dot) pc(-2) in star-forming regions within nearby galaxies.