{"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/dependence-of-x-rm-co-on-metallicity","title":"Dependence of $X_{\\rm CO}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium","arxiv_id":"2201.03885","date":"2022-01-11","proceeding":null,"authors":["Chia-Yu Hu","Andreas Schruba","Amiel Sternberg","Ewine F. van Dishoeck"],"abstract":"We study the CO(1-0)-to-H$_2$ conversion factor ($X_{\\rm CO}$) and the line ratio of CO(2-1)-to-CO(1-0) ($R_{21}$) across a wide range of metallicity ($0.1 \\leq Z/Z_\\odot \\leq 3$) in high-resolution (~0.2 pc) hydrodynamical simulations of a self-regulated multiphase interstellar medium. We construct synthetic CO emission maps via radiative transfer and systematically vary the \"observational\" beam size to quantify the scale dependence. We find that the kpc-scale $X_{\\rm CO}$ can be over-estimated at low $Z$ if assuming steady-state chemistry or assuming that the star-forming gas is H$_2$-dominated. On parsec scales, $X_{\\rm CO}$ varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The pc-scale $X_{\\rm CO}$ drops to the Milky Way value of $2\\times 10^{20}\\ {\\rm cm^{-2}~(K~km~s^{-1})^{-1}}$ once dust shielding becomes effective, independent of $Z$. The CO lines become increasingly optically thin at lower $Z$, leading to a higher $R_{21}$. Most cloud area is filled by diffuse gas with high $X_{\\rm CO}$ and low $R_{21}$, while most CO emission originates from dense gas with low $X_{\\rm CO}$ and high $R_{21}$. Adopting a constant $X_{\\rm CO}$ strongly over- (under-)estimates H$_2$ in dense (diffuse) gas. The line intensity negatively (positively) correlates with $X_{\\rm CO}$ ($R_{21}$) as it is a proxy of column density (volume density). On large scales, $X_{\\rm CO}$ and $R_{21}$ are dictated by beam averaging, and they are naturally biased towards values in dense gas. Our predicted $X_{\\rm CO}$ is a multivariate function of $Z$, line intensity, and beam size, which can be used to more accurately infer the H$_2$ mass.","url_abs":"https://arxiv.org/abs/2201.03885v3","url_pdf":"https://arxiv.org/pdf/2201.03885v3.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":"dependence-of-x-rm-co-on-metallicity","repo_url":"https://github.com/huchiayu/particlegridmapper.jl","is_official":1,"mentioned_in_paper":1,"mentioned_in_github":1,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"syntology_url":null,"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}