Possibility of Using 3.3 μm Polycyclic Aromatic Hydrocarbon 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 μ m polycyclic aromatic hydrocarbon (PAH) emission flux or its upper limit is available. A scaling relation between 3.3 μ 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: ${\mathrm{log}}\,{L}_{3.3}/{L}_{\odot }$ = $(1.00\pm 0.07)\times {\rm{log}}\,{L}_{{\rm{CO}}(1-0)}^{{\prime} }/({\rm{K}}\,{\rm{km}}\,{{\rm{s}}}^{-1}\,{{\rm{pc}}}^{2})$ +(−1.10 ± 0.70). The slope is near unity, allowing the use of a single value of $\langle {\rm{log}}\,({L}_{3.3}/{L}_{{\rm{CO}}(1-0)}^{{\prime} })\rangle $ =−1.09 ± 0.36 [ L _⊙ /(K km s ^−1 pc ^2 )] in the conversion between 3.3 μ m PAH and CO luminosities. The variation in the ${L}_{3.3}/{L}_{{\rm{CO}}}^{{\prime} }$ ratio is not dependent on the galaxy properties, including total IR luminosity, stellar mass, and star formation rate excess. The total gas mass, estimated using the dust-to-gas ratio and dust mass, is correlated with 3.3 μ m PAH luminosity, in line with the prescription using α _CO = 0.8–4.5 covering both normal star-forming galaxies and starburst galaxies. Active galactic nucleus (AGN)-dominated galaxies tend to have a lower ${L}_{3.3}/{L}_{{\rm{CO}}}^{{\prime} }$ than non-AGN galaxies, which needs to be investigated further with an increased sample size. The established L _3.3 – ${L}_{{\rm{CO}}}^{{\prime} }$ correlation is expected to be applicable to wide-field near-infrared spectrophotometric surveys that allow the detection of 3.3 μ m emission from numerous low-redshift galaxies.

Keywords

• Luminous infrared galaxies
• Ultraluminous infrared galaxies
• Starburst galaxies
• Polycyclic aromatic hydrocarbons
• Molecular gas
• Star formation