The Correlation between WISE 12 μm Emission and Molecular Gas Tracers on Subkiloparsec Scales in Nearby Star-forming Galaxies

Abstract

Abstract We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of 12CO and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 μm emission and molecular gas tracers at subkiloparsec scales. We find that 12 μm luminosity is more tightly correlated with 12CO than it is with 13CO or dense gas tracers. Residuals between predicted and observed 12CO are only weakly correlated with molecular gas mass surface density (Σmol) in regions where Σmol is very low (∼10 M ⊙ pc−2). Above this limit, the 12CO residuals show no correlations with physical conditions of molecular gas, while 13CO residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the 12CO−12 μm relation is strong and statistically robust with respect to star-forming galaxies and active galactic nucleus hosts. These results suggest that WISE 12 μm emission can be used to trace total molecular gas instead of dense molecular gas, likely because polycyclic aromatic hydrocarbons (PAHs, a major contributor to WISE 12 μm emission) may be well mixed with the gas that is traced by 12CO. We propose that WISE 12 μm luminosity can be used to estimate molecular gas surface density for statistical analyses of the star formation process in galaxies.