Possibility of using 3.3$\mu$m PAH luminosity as a molecular gas mass estimator

Abstract

We present CO(1-0) observations of 50 star-forming galaxies at 0.01<z<0.35, for which 3.3$\mu$m PAH emission flux or its upper limit is available. A scaling relation between 3.3$\mu$m PAH luminosity and CO(1-0) luminosity is established covering ~2 orders of magnitude in total IR luminosity and CO luminosity, with a scatter of ~0.23 dex: $\log L_{3.3}/{\mathrm{L}}_{\odot }=(1.00\pm 0.07)\times \log L_{\mathrm{CO}(1-0)}^{\prime }/(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2)+(-1.10\pm 0.70)$. The slope is near unity, allowing the use of a single value of $⟨\log (L_{3.3}/L_{\mathrm{CO}(1-0)}^{\prime })⟩=-1.09\pm 0.36[{\mathrm{L}}_{\odot }/(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\mathrm{p}{\mathrm{c}}^2)]$ in the conversion between 3.3$\mu$m PAH and CO luminosities. The variation in the $L_{3.3}/L_{\mathrm{CO}}^{\prime }$ ratio is not dependent on the galaxy properties, including total IR luminosity, stellar mass, and SFR excess. The total gas mass, estimated using dust-to-gas ratio and dust mass, is correlated with 3.3$\mu$m PAH luminosity, in line with the prescription using ${\alpha }_{\mathrm{CO}}=0.8-4.5$ covering both normal star-forming galaxies and starburst galaxies. AGN-dominated galaxies tend to have a lower $L_{3.3}/L_{\mathrm{CO}}^{\prime }$ than non-AGN galaxies, which needs to be investigated further with an increased sample size. The established $L_{3.3}$-$L_{\mathrm{CO}}^{\prime }$ correlation is expected to be applicable to wide-field near-infrared spectrophotometric surveys that allow the detection of 3.3$\mu$m emission from numerous low-redshift galaxies.

19 pages, 8 figures, 3 tables, Accepted for publication in ApJ