Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H 2 Conversion Factor Across Thousands of Local Galaxies

Journal article

We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density (Σ⋆) and star formation rate surface density (ΣSFR), and predict the CO-to-H2 conversion factor (αCO) for over 5000 local galaxies with stellar mass M⋆ ≥ 109.3 M⊙. We build these profiles and measure galaxy half-light radii using GALEX and WISE images from the z0MGS program, with special care given to highly inclined galaxies. From the UV and IR surface brightness profiles, we estimate Σ⋆ and ΣSFR and use them to predict αCO with state-of-the-art empirical prescriptions. We validate our (kpc-scale) αCO 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 αCO 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⋆ = 109.5–11 M⊙ show strong anticorrelations of SFR/ L′CO(1–0)∝M⋆−0.29 and SFR/ L′CO(2–1)∝M⋆−0.40 . The estimated αCO trends, when combined with a constant molecular gas depletion time tdep, can only explain ≈1/3 of these SFR/ L′CO trends. This suggests that tdep 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⋆, ΣSFR, and αCO estimates.

Authors: Sun, J; Teng, Y; Chiang, I; Leroy, AK; Sandstrom, K

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal
• Volume: 994
• Issue: 2
• Article number: 263
• Publication date: 2025-12-01
• Acceptance date: 2025-10-04
• DOI: 10.3847/1538-4357/ae10be
• EISSN: 1538-4357
• ISSN: 0004637X, 0004-637X
• Language: English
• Keywords: Disk galaxies; CO line emission; Star formation; Molecular gas