Dependence of X CO on Metallicity, Intensity, and Spatial Scale in a Self-regulated Interstellar Medium
Abstract
We study the CO(1-0)-to-H2 conversion factor (X CO) and the line ratio of CO(2-1)-to-CO(1-0) (R 21) across a wide range of metallicity (0.1 ≤ Z/Z ⊙ ≤ 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 CO can be overestimated at low Z if assuming steady-state chemistry or assuming that the star-forming gas is H2 dominated. On parsec scales, X CO varies by orders of magnitude from place to place, primarily driven by the transition from atomic carbon to CO. The parsec-scale X CO drops to the Milky Way value of $2\times {10}^{20}\ {\mathrm{cm}}^{-2}\,{\left({\rm{K}}\,\mathrm{km}\,{{\rm{s}}}^{-1}\right)}^{-1}$ 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 CO and low R 21, while most CO emission originates from dense gas with low X CO and high R 21. Adopting a constant X CO strongly over- (under-)estimates H2 in dense (diffuse) gas. The line intensity negatively (positively) correlates with X CO (R 21) as it is a proxy of column density (volume density). On large scales, X CO and R 21 are dictated by beam averaging, and they are naturally biased toward values in dense gas. Our predicted X CO is a multivariate function of Z, line intensity, and beam size, which can be used to more accurately infer the H2 mass.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2022
- DOI:
- 10.3847/1538-4357/ac65fd
- arXiv:
- arXiv:2201.03885
- Bibcode:
- 2022ApJ...931...28H
- Keywords:
-
- Interstellar medium;
- Astrochemistry;
- Hydrodynamical simulations;
- 847;
- 75;
- 767;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Instrumentation and Methods for Astrophysics
- E-Print:
- 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