{"about":{"site":"https://codewithpapers.app","non_affiliation":"Code with Papers and Syntology are not affiliated with, endorsed by, or sponsored by Papers with Code, Meta, or the pwc-archive mirror.","licence":"CC BY-SA 4.0","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","attribution":"https://codewithpapers.app/attribution","modified":"archive material modified by Syntology; see the attribution page"},"url":"/paper/the-physical-drivers-and-observational","title":"The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers","arxiv_id":"2304.04732","date":"2023-04-10","proceeding":null,"authors":["Yu-Hsuan Teng","Karin M. Sandstrom","Jiayi Sun","Munan Gong","Alberto D. Bolatto","I-Da Chiang","Adam K. Leroy","Antonio Usero","Simon C. O. Glover","Ralf S. Klessen","Daizhong Liu","Miguel Querejeta","Eva Schinnerer","Frank Bigiel","Yixian Cao","Melanie Chevance","Cosima Eibensteiner","Kathryn Grasha","Frank P. Israel","Eric J. Murphy","Lukas Neumann","Hsi-An Pan","Francesca Pinna","Mattia C. Sormani","J. D. T. Smith","Fabian Walter","Thomas G. Williams"],"abstract":"The CO-to-H$_2$ conversion factor ($\\alpha_\\rm{CO}$) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower $\\alpha_\\rm{CO}$ in the centers of some barred galaxies on kpc scales. To unveil the physical drivers of such variations, we obtained ALMA Band 3, 6, and 7 observations toward the inner 2 kpc of NGC 3627 and NGC 4321 tracing $^{12}$CO, $^{13}$CO, and C$^{18}$O lines on 100 pc scales. Our multi-line modeling and Bayesian likelihood analysis of these datasets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of $\\alpha_\\rm{CO}$. The central 300 pc nuclei in both galaxies show strong enhancement of temperature $T_\\rm{k}>100$ K and density $n_\\rm{H_2}>10^3$ cm$^{-3}$. Assuming a CO-to-H$_2$ abundance of $3\\times10^{-4}$, we derive 4-15 times lower $\\alpha_\\rm{CO}$ than the Galactic value across our maps, which agrees well with previous kpc-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of $\\alpha_\\rm{CO}$ with low-J $^{12}$CO optical depths ($\\tau_\\rm{CO}$), as well as an anti-correlation with $T_\\rm{k}$. The $\\tau_\\rm{CO}$ correlation explains most of the $\\alpha_\\rm{CO}$ variation in the three galaxy centers, whereas changes in $T_\\rm{k}$ influence $\\alpha_\\rm{CO}$ to second order. Overall, the observed line width and $^{12}$CO/$^{13}$CO 2-1 line ratio correlate with $\\tau_\\rm{CO}$ variation in these centers, and thus they are useful observational indicators for $\\alpha_\\rm{CO}$ variation. We also test current simulation-based $\\alpha_\\rm{CO}$ prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.","url_abs":"https://arxiv.org/abs/2304.04732v1","url_pdf":"https://arxiv.org/pdf/2304.04732v1.pdf","source":{"archive":"pwc-archive (Hugging Face), CC BY-SA 4.0","snapshot":"2025-07-28","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","row_kind":"links_only","authors_date_abstract":"arXiv metadata, CC0 1.0 (https://info.arxiv.org/help/license), from the Kaggle arXiv metadata snapshot of 2026-09-12"},"code_links":[{"paper_slug":"the-physical-drivers-and-observational","repo_url":"https://github.com/elthateng/multiline-bayesian-modeling","is_official":1,"mentioned_in_paper":1,"mentioned_in_github":1,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}