PAHs as tracers of the molecular gas in star-forming galaxies

Abstract

We combine new CO(1-0) line observations of 24 intermediate redshift galaxies (0.03 < z < 0.28) along with literature data of galaxies at 0 < z < 4 to explore scaling relations between the dust and gas content using polycyclic aromatic hydrocarbon (PAH) 6.2 μm (L_6.2), CO (L^' }_CO), and infrared (L_IR) luminosities for a wide range of redshifts and physical environments. Our analysis confirms the existence of a universal L_6.2-L^' }_CO correlation followed by normal star-forming galaxies (SFGs) and starbursts (SBs) at all redshifts. This relation is also followed by local ultraluminous infrared galaxies that appear as outliers in the L_6.2-L_IR and L_IR-L^' }_CO relations defined by normal SFGs. The emerging tight (σ ≈ 0.26 dex) and linear (α = 1.03) relation between L_6.2 and L^' }_CO indicates a L_6.2 to molecular gas (M_{H_2}) conversion factor of α_6.2 = M_{H_2}/L_6.2 = (2.7 ± 1.3) × α_CO, where α_CO is the L^' }_CO to M_{H_2} conversion factor. We also find that on galaxy integrated scales, PAH emission is better correlated with cold rather than with warm dust emission, suggesting that PAHs are associated with the diffuse cold dust, which is another proxy for M_{H_2}. Focusing on normal SFGs among our sample, we employ the dust continuum emission to derive M_{H_2} estimates and find a constant M_{H_2}/L_6.2 ratio of α_6.2 = 12.3 M_{⊙}/L_⊙(σ ≈ 0.3 dex). This ratio is in excellent agreement with the L^' }_CO-based M_{H_2}/L_6.2 values for α_CO = 4.5 M_{⊙ }/(K km s^-1 pc²) which is typical of normal SFGs. We propose that the presented L_6.2-L^' }_CO and L_6.2-M_{H_2} relations will serve as useful tools for the determination of the physical properties of high-z SFGs, for which PAH emission will be routinely detected by the James Webb Space Telescope.