Dependence of $X_{\mathrm{CO}}$ on metallicity, intensity, and spatial scale in a self-regulated interstellar medium

Abstract

We study the CO(1-0)-to-H${}_2$ conversion factor ($X_{\mathrm{CO}}$) and the line ratio of CO(2-1)-to-CO(1-0) ($R_{21}$) across a wide range of metallicity ($0.1\le Z/Z_{\odot }\le 3$) 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 $X_{\mathrm{CO}}$ 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, $X_{\mathrm{CO}}$ varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The pc-scale $X_{\mathrm{CO}}$ drops to the Milky Way value of $2\times 10^{20}\mathrm{c}{\mathrm{m}}^{-2}(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$ 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 $X_{\mathrm{CO}}$ and low $R_{21}$, while most CO emission originates from dense gas with low $X_{\mathrm{CO}}$ and high $R_{21}$. Adopting a constant $X_{\mathrm{CO}}$ strongly over- (under-)estimates H${}_2$ in dense (diffuse) gas. The line intensity negatively (positively) correlates with $X_{\mathrm{CO}}$ ($R_{21}$) as it is a proxy of column density (volume density). On large scales, $X_{\mathrm{CO}}$ and $R_{21}$ are dictated by beam averaging, and they are naturally biased towards values in dense gas. Our predicted $X_{\mathrm{CO}}$ is a multivariate function of $Z$, line intensity, and beam size, which can be used to more accurately infer the H${}_2$ mass.

Published in ApJ (typos in the abstract fixed). Code for interpolating Lagrangian (particle) data onto an adaptive mesh (and auxiliary scripts for RADMC-3D) available at https://github.com/huchiayu/ParticleGridMapper.jl

References in corpus (17)

• Theory of Star Formation
• The Resolved Properties of Extragalactic Giant Molecular Clouds
• Simulating the formation of molecular clouds. I. Slow formation by gravitational collapse from static initial conditions
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• Herschel Survey of Galactic OH+, H2O+, and H3O+: Probing the Molecular Hydrogen Fraction and Cosmic-Ray Ionization Rate
• Tracing the total molecular gas in galaxies: [CII] and the CO-dark gas
• New Constraints on the ${}^{12}$CO(2-1)/(1-0) Line Ratio Across Nearby Disc Galaxies
• The Cloud Factory I: Generating resolved filamentary molecular clouds from galactic-scale forces
• Physical Properties of Molecular Clouds at 2 parsec Resolution in the Low-Metallicity Dwarf Galaxy NGC 6822 and the Milky Way
• The environmental dependence of the X_CO conversion factor
• Metallicity dependence of the H/H${}_2$ and C${}^{+}$/C/CO distributions in a resolved self-regulating interstellar medium
• Using CO line ratios to trace the physical properties of molecular clouds
• A Schmidt-Kennicutt law for star formation in the Milky Way disk
• Photodissociation Region Diagnostics Across Galactic Environments
• The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A
• Synthetic CO, H2 and HI surveys of the Galactic 2nd Quadrant, and the properties of molecular gas
• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds

Cited by in corpus (16)

• What Causes The Formation of Disks and End of Bursty Star Formation?
• Introducing TIGRESS-NCR: I. Co-Regulation of the Multiphase Interstellar Medium and Star Formation Rates
• Photochemistry and Heating/Cooling of the Multiphase Interstellar Medium with UV Radiative Transfer for Magnetohydrodynamic Simulations
• The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers
• Co-evolution of Dust and Chemistry in Galaxy Simulations with a Resolved Interstellar Medium
• Evidence for extended gaseous reservoirs around AGN at cosmic noon from ALMA CO(3-2) observations
• Gas, dust, and the CO-to-molecular gas conversion factor in low-metallicity starbursts
• Metallicity dependence of the CO-to-H${}_2$ and the [CI]-to-H${}_2$ conversion factors in galaxies
• First Detection of Radio Recombination Lines of Ions Heavier than Helium
• Gravitationally Bound Gas Determines Star Formation in the Galaxy
• The relationship between cluster environment and molecular gas content of star-forming galaxies in the EAGLE simulation
• Exploring extreme conditions for star formation: a deep search for molecular gas in the Leo ring
• A Tentative Detection of Molecular Hydrogen (H${}_2$) Emission Lines at Cosmic Dawn
• Molecular gas and star formation in central rings across nearby galaxies
• Does the HCN/CO ratio trace the star-forming fraction of gas? II. Variations in CO and HCN Emissivity
• Resolved molecular gas and star-formation in massive unquenched spirals : I. UGC 8179