Abstract We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density (Σ ⋆ ) and star formation rate surface density (Σ SFR ), and predict the CO-to-H 2 conversion factor ( α CO ) for over 5000 local galaxies with stellar mass M ⋆ ≥ 10 9.3 M ⊙ . We build these profiles and measure galaxy half-light radii using GALEX and WISE images from the z 0MGS program, with special care given to highly inclined galaxies. From the UV and IR surface brightness profiles, we estimate Σ ⋆ and Σ SFR and use them to predict α CO with state-of-the-art empirical prescriptions. We validate our (kpc-scale) α CO predictions against observational estimates, finding the best agreement when accounting for CO-dark gas as well as CO emissivity and excitation effects. The CO-dark correction plays a primary role in lower-mass galaxies, whereas CO emissivity and excitation effects become more important in higher-mass and more actively star-forming galaxies, respectively. We compare our estimated α CO to observed galaxy-integrated SFR to CO luminosity ratio as a function of M ⋆ . A large compilation of literature data suggests that star-forming galaxies with M ⋆ = 10 9.5–11 M ⊙ show strong anticorrelations of SFR/ L ′ CO ( 1 – 0 ) ∝ M ⋆ − 0.29 and SFR/ L ′ CO ( 2 – 1 ) ∝ M ⋆ − 0.40 . The estimated α CO trends, when combined with a constant molecular gas depletion time t dep , can only explain ≈1/3 of these SFR/ L ′ CO trends. This suggests that t dep being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/ L ′ CO variations. We publish all data products from this work, including galaxy sizes, UV and IR surface brightness profiles, Σ ⋆ , Σ SFR , and α CO estimates.
Chiang et al. (Fri,) studied this question.
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