Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H$_2$ Conversion Factor Across Thousands of Local Galaxies
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| Format: | Preprint |
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2025
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| author | Sun, Jiayi Teng, Yu-Hsuan Chiang, I-Da Leroy, Adam K. Sandstrom, Karin Brok, Jakob den Bolatto, Alberto D. Chastenet, Jeremy Chown, Ryan Hughes, Annie Koch, Eric W. Williams, Thomas G. |
| author_facet | Sun, Jiayi Teng, Yu-Hsuan Chiang, I-Da Leroy, Adam K. Sandstrom, Karin Brok, Jakob den Bolatto, Alberto D. Chastenet, Jeremy Chown, Ryan Hughes, Annie Koch, Eric W. Williams, Thomas G. |
| contents | We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density ($Σ_\star$) and star formation rate surface density ($Σ_\mathrm{SFR}$), and predict the CO-to-H$_2$ conversion factor ($α_\mathrm{CO}$) for over 5,000 local galaxies with stellar mass $M_\star\,{\geq}\,10^{9.3}\rm\,M_\odot$. 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 $Σ_\star$ and $Σ_\mathrm{SFR}$ and use them to predict $α_\mathrm{CO}$ with state-of-the-art empirical prescriptions. We validate our (kpc-scale) $α_\mathrm{CO}$ 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 $α_\mathrm{CO}$ 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/$L^\prime_\mathrm{CO(1{-}0)} \propto M_\star^{-0.29}$ and SFR/$L^\prime_\mathrm{CO(2{-}1)} \propto M_\star^{-0.40}$. The estimated $α_\mathrm{CO}$ trends, when combined with a constant molecular gas depletion time $t_\mathrm{dep}$, can only explain ${\approx}1/3$ of these SFR/$L^\prime_\mathrm{CO}$ trends. This suggests that $t_\mathrm{dep}$ being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/$L^\prime_\mathrm{CO}$ variations. (Abridged) |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2510_05214 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H$_2$ Conversion Factor Across Thousands of Local Galaxies Sun, Jiayi Teng, Yu-Hsuan Chiang, I-Da Leroy, Adam K. Sandstrom, Karin Brok, Jakob den Bolatto, Alberto D. Chastenet, Jeremy Chown, Ryan Hughes, Annie Koch, Eric W. Williams, Thomas G. Astrophysics of Galaxies We present radial profiles of surface brightness in UV and IR bands, estimate stellar mass surface density ($Σ_\star$) and star formation rate surface density ($Σ_\mathrm{SFR}$), and predict the CO-to-H$_2$ conversion factor ($α_\mathrm{CO}$) for over 5,000 local galaxies with stellar mass $M_\star\,{\geq}\,10^{9.3}\rm\,M_\odot$. 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 $Σ_\star$ and $Σ_\mathrm{SFR}$ and use them to predict $α_\mathrm{CO}$ with state-of-the-art empirical prescriptions. We validate our (kpc-scale) $α_\mathrm{CO}$ 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 $α_\mathrm{CO}$ 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/$L^\prime_\mathrm{CO(1{-}0)} \propto M_\star^{-0.29}$ and SFR/$L^\prime_\mathrm{CO(2{-}1)} \propto M_\star^{-0.40}$. The estimated $α_\mathrm{CO}$ trends, when combined with a constant molecular gas depletion time $t_\mathrm{dep}$, can only explain ${\approx}1/3$ of these SFR/$L^\prime_\mathrm{CO}$ trends. This suggests that $t_\mathrm{dep}$ being systematically shorter in lower-mass star-forming galaxies is the main cause of the observed SFR/$L^\prime_\mathrm{CO}$ variations. (Abridged) |
| title | Resolved Profiles of Stellar Mass, Star Formation Rate, and Predicted CO-to-H$_2$ Conversion Factor Across Thousands of Local Galaxies |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2510.05214 |