  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.
