2022ApJ...940..133G

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 (Σ_mol) in regions where Σ_mol 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.