A new estimator of resolved molecular gas in nearby galaxies

ABSTRACT 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 μm 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 μm images. We find strong CO-12 μm correlations within each galaxy and we show that the scatter in the global CO-12 μm correlation is largely driven by differences from galaxy to galaxy. The correlation is stronger than that between star formation rate and H2 surface densities [Σ(H2)]. We explore multivariable regression to predict Σ(H2) in star-forming pixels using the WISE 12 μm data combined with global and resolved galaxy properties, and provide the fit parameters for the best estimators. We find that Σ(H2) estimators that include $\Sigma (\mathrm{12\:\mu m})$ are able to predict Σ(H2) more accurately than estimators that include resolved optical properties instead of $\Sigma (\mathrm{12\:\mu m})$. These results suggest that 12 μm 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 H2. The best single-property estimator is $\log \frac{\Sigma (\mathrm{H_2})}{\mathrm{M_\odot \:pc^{-2}}} = (0.48 \pm 0.01) + (0.71 \pm 0.01)\log \frac{\Sigma (\mathrm{12\:\mu m})}{\mathrm{L_\odot \:pc^{-2}}}$. This correlation can be used to efficiently estimate Σ(H2) down to at least 1 M⊙ pc−2 in star-forming regions within nearby galaxies.