The Correlation between WISE 12 μm Emission and Molecular Gas Tracers on Subkiloparsec Scales in Nearby Star-forming Galaxies
Yang Gao, Qinghua Tan, Yu Gao, Min Fang, Ryan Chown, Qian Jiao, Chunsheng Luo
Abstract
Abstract We complement the MALATANG sample of dense gas in nearby galaxies with archival observations of 12 CO and its isotopologues to determine scaling relations between Wide-field Infrared Survey Explorer (WISE) 12 μ m emission and molecular gas tracers at subkiloparsec scales. We find that 12 μ m luminosity is more tightly correlated with 12 CO than it is with 13 CO or dense gas tracers. Residuals between predicted and observed 12 CO 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 12 CO residuals show no correlations with physical conditions of molecular gas, while 13 CO residuals depend on the gas optical depth and temperature. By analyzing differences from galaxy to galaxy, we confirm that the 12 CO−12 μ m relation is strong and statistically robust with respect to star-forming galaxies and active galactic nucleus hosts. These results suggest that WISE 12 μ 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 μ m emission) may be well mixed with the gas that is traced by 12 CO. We propose that WISE 12 μ m luminosity can be used to estimate molecular gas surface density for statistical analyses of the star formation process in galaxies.