The correlation between WISE 12 $\mu$m emission and molecular gas tracers on sub-kpc scales in nearby star-forming galaxies

Abstract

We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of ${}^{12}\mathrm{CO}$ and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 $\mu$m emission and molecular gas tracers at sub-kiloparsec scales. We find that 12 $\mu$m luminosity is more tightly correlated with ${}^{12}\mathrm{CO}$ than it is with ${}^{13}\mathrm{CO}$ or dense gas tracers. Residuals between predicted and observed ${}^{12}\mathrm{CO}$ are only weakly correlated with molecular gas mass surface density (${\Sigma }_{\mathrm{mol}}$) in regions where ${\Sigma }_{\mathrm{mol}}$ is very low ($\sim 10{\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$). Above this limit, the ${}^{12}\mathrm{CO}$ residuals show no correlations with physical conditions of molecular gas, while ${}^{13}\mathrm{CO}$ residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the ${}^{12}\mathrm{CO}$-12 $\mu$m relation is strong and statistically robust with respect to star forming galaxies and AGN 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 ${}^{12}\mathrm{CO}$. 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.

16 pages, 7 figures, accepted for publication in ApJ

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