Estimating Molecular Gas Content in Galaxies from PAH Emission

TL;DR

This study demonstrates that PAH emission in galaxies correlates strongly with molecular gas content, enabling a new empirical method to estimate molecular gas mass using mid-infrared observations with minimal scatter.

Contribution

It introduces an empirical calibration linking PAH emission to molecular gas mass, validated on spatially resolved data across diverse galaxy environments.

Findings

PAH emission correlates tightly with CO emission on sub-kpc scales.

The calibration achieves a scatter of about 0.2-0.25 dex in estimating gas mass.

Mid-infrared PAH-sensitive bands can substitute for direct CO measurements.

Abstract

Emission from polycyclic aromatic hydrocarbons (PAHs), a commonly used indicator of star formation activity in galaxies, also has the potential to serve as an effective empirical tracer of molecular gas. We use a sample of 19 nearby galaxies with spatially resolved mid-infrared Spitzer spectroscopy, multi-wavelength optical and mid-infrared imaging, and millimeter interferometric CO(1-0) maps to investigate the feasibility of using PAH emission as an empirical proxy to estimate molecular gas mass. PAH emission correlates strongly with CO emission on sub-kpc scales over the diverse environments probed by our sample of star-forming galaxies and low-luminosity active galactic nuclei. The tight observed correlation, likely a consequence of photoelectronic heating of the diffuse interstellar gas by the PAHs, permits us to derive an empirical calibration to estimate molecular gas mass from the luminosity of PAH emission that has a total scatter of only {\sim} 0.2 - 0.25 dex. Mid-infrared bands sensitive to PAH emission (e.g., the Spitzer/IRAC4 and WISE/W3 filters) can also be used as a highly effective substitute for this purpose.