Metallicity dependence of the CO-to-H₂ and the [CI]-to-H₂ conversion factors in galaxies

¹ Research Center for Astronomical Computing, Zhejiang Laboratory, Hangzhou 311000, China
² School of Astronomy and Space Science, Nanjing University, Nanjing 210093, China
³ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China
⁴ Department of Physics, Aristotle University of Thessaloniki, Greece
⁵ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, 38400 Saint-Martin-d’Héres, France
⁶ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
⁷ Key Laboratory of Radio Astronomy and Technology (Chinese Academy of Sciences), A20 Datun Road, Chaoyang District, Beijing 100101, PR China
⁸ I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
⁹ New Cornerstone Science Laboratory, Department of Astronomy, Tsinghua University, Beijing 100084, China
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

^★ Corresponding authors: tbisbas@zhejianglab.com; zzhang@nju.edu.cn

Received: 7 February 2025
Accepted: 14 March 2025

Abstract

Understanding the molecular gas content in the interstellar medium (ISM) is crucial for studying star formation and galaxy evolution. The CO-to-H₂ (X_CO) and the [CI]-to-H₂ (X_CI) conversion factors are widely used to estimate the molecular mass content in galaxies. However, these factors depend on many environmental parameters in the ISM, such as metallicity, cosmic-ray ionization rate, and far-ultraviolet (FUV) radiation field, in particular, in the low-metallicity ISM that is found at large galactocentric radii and in early-type galaxies. This work investigates the dependence of X_CO and X_CI on the environmental parameters of the ISM, with a focus on the low-metallicity α-enhanced ISM ([C/O] < 0), to provide improved tracers of molecular gas under diverse conditions. We used the statistical algorithm PDFCHEM, coupled with a database of photodissociation region (PDR) models generated with the 3D-PDR astrochemical code. The models account for a wide range of metallicities, dust-to-gas mass ratios, FUV intensities, and cosmic-ray ionization rates. The conversion factors were computed by integrating the PDR properties over log-normal column density distributions (A_V-PDFs) that represent various cloud types. The X_CO factor increases significantly with decreasing metallicity. It exceeds ∼1000 times the Galactic value at [O/H] = −1.0 under α-enhanced conditions, as opposed to ∼300 times under non-α-enhanced conditions ([C/O] = 0). In contrast, X_CI varies more gradually with metallicity, which makes it a more reliable tracer of molecular gas in metal-poor environments under most conditions. The fraction of CO-dark molecular gas increases dramatically in low-metallicity regions, where it exceeds 90% at [O/H] = −1.0, in particular, in diffuse clouds and environments with strong FUV radiation fields. The results highlight the limitations of CO as a molecular gas tracer in the metal-poor ISM and demonstrate the potential of [CI] (1–0) as a complementary tracer. The use of metallicity-dependent X_CO and X_CI factors as provided by this study is recommended for accurately estimating molecular gas masses in diverse environments. We recommend the use of the log₁₀ X_CO ≃ −2.41 Z + 41.3 relation for the CO-to-H₂ conversion factor and the log₁₀ X_CI ≃ −0.99 Z + 29.7 relation for the [CI]-to-H₂ conversion factor, where Z = 12 + log₁₀(O/H).