The First Exploration of the Correlations Between WISE 12 μm and CO Emission in Early-type Galaxies

Abstract

We present the analysis of a comprehensive sample of 352 early-type galaxies using public data, to investigate the correlations between CO luminosities and mid-infrared luminosities observed by Wide-field Infrared Survey Explorer. We find strong correlations between both CO (1–0) and CO (2–1) luminosities and 12 μ m luminosity, boasting a correlation coefficient greater than 0.9 and an intrinsic scatter smaller than 0.1 dex. The consistent slopes observed for the relationships of CO (1–0) and CO (2–1) suggest that the line ratio R 21 lacks correlation with mid-infrared emission in early-type galaxies, which is significantly different from star-forming galaxies. Moreover, the slopes of L _CO(1−0) – L _12 _μ _m and L _CO(2−1) – L _12 _ μ _m relations in early-type galaxies are steeper than those observed in star-forming galaxies. Given the absence of correlation with color, morphology, or specific star formation rate (sSFR), the correlation between deviations and the molecular gas mass surface density could be eliminated by correcting the possible 12 μ m emission from old stars or adopting a systematically different α _CO . The latter, on average, is equivalent to adding a constant CO brightness density, specifically $2.{8}_{-0.6}^{+0.8}\,[{\rm{K}}\,{\rm{km}}\,{{\rm{s}}}^{-1}]$ and $4.{4}_{-1.4}^{+2.2}\,[{\rm{K}}\,{\rm{km}}\,{{\rm{s}}}^{-1}]$ for CO (1–0) and (2–1), respectively. These explorations will serve as useful tools for estimating the molecular gas content in gas-poor galaxies and understanding associated quenching processes.

Keywords

• Galaxy evolution
• Interstellar medium
• Molecular gas
• Infrared photometry
• Elliptical galaxies