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

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