Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H${}_2$ Conversion Factor Across Thousands of Local Galaxies

Abstract

We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density (${\Sigma }_{\star }$) and star formation rate surface density (${\Sigma }_{\mathrm{SFR}}$), and predict the CO-to-H${}_2$ conversion factor (${\alpha }_{\mathrm{CO}}$) for over 5,000 local galaxies with stellar mass $M_{\star }\ge 10^{9.3}{\mathrm{M}}_{\odot }$. 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 ${\Sigma }_{\star }$ and ${\Sigma }_{\mathrm{SFR}}$ and use them to predict ${\alpha }_{\mathrm{CO}}$ with state-of-the-art empirical prescriptions. We validate our (kpc-scale) ${\alpha }_{\mathrm{CO}}$ 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 ${\alpha }_{\mathrm{CO}}$ to observed galaxy-integrated SFR to CO luminosity ratio as a function of $M_{\star }$. A large compilation of literature data suggests that star-forming galaxies with $M_{\star }=10^{9.5-11}{M}_{\odot }$ show strong anti-correlations of SFR/$L_{\mathrm{CO}(1-0)}^{\prime }\propto M_{\star }^{-0.29}$ and SFR/$L_{\mathrm{CO}(2-1)}^{\prime }\propto M_{\star }^{-0.40}$. The estimated ${\alpha }_{\mathrm{CO}}$ trends, when combined with a constant molecular gas depletion time $t_{\mathrm{dep}}$, can only explain $\approx 1/3$ of these SFR/$L_{\mathrm{CO}}^{\prime }$ trends. This suggests that $t_{\mathrm{dep}}$ being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/$L_{\mathrm{CO}}^{\prime }$ variations. (Abridged)

16 pages main text + 4 appendices. ApJ in press. We publish all data products (including galaxy sizes, UV and IR surface brightness profiles, stellar mass and SFR profiles, and conversion factor estimates) at https://www.canfar.net/storage/vault/list/z0MGS/Sun_etal_2025