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

TL;DR

This study demonstrates that WISE 12 μm emission correlates strongly with $^{12}$CO molecular gas tracers on sub-kiloparsec scales in nearby star-forming galaxies, enabling estimation of molecular gas surface density.

Contribution

It establishes that WISE 12 μm emission is a reliable tracer of total molecular gas, not just dense gas, across different galaxy types and conditions.

Findings

12 μm luminosity correlates more tightly with $^{12}$CO than with $^{13}$CO or dense gas tracers.

Residuals between predicted and observed $^{12}$CO are weakly affected by molecular gas surface density at low densities.

The $^{12}$CO-12 μm relation is consistent across star-forming and AGN-host galaxies.

Abstract

We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of $^{12}\rm CO$ 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 $^{12}\rm CO$ than it is with $^{13}\rm CO$ or dense gas tracers. Residuals between predicted and observed $^{12}\rm CO$ are only weakly correlated with molecular gas mass surface density ($\Sigma_{\rm mol}$) in regions where $\Sigma_{\rm mol}$ is very low ($\sim 10~{\rm M_{\odot}~pc^{-2}}$). Above this limit, the $^{12}\rm CO$ residuals show no correlations with physical conditions of molecular gas, while $^{13}\rm CO$ residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the $^{12}\rm CO$-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 $^{12}\rm CO$. 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.