Dependence of X _{CO} on Metallicity, Intensity, and Spatial Scale in a Selfregulated Interstellar Medium
Abstract
We study the CO(10)toH_{2} conversion factor (X _{CO}) and the line ratio of CO(21)toCO(10) (R _{21}) across a wide range of metallicity (0.1 ≤ Z/Z _{⊙} ≤ 3) in highresolution (~0.2 pc) hydrodynamical simulations of a selfregulated 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 kpcscale X _{CO} can be overestimated at low Z if assuming steadystate chemistry or assuming that the starforming gas is H_{2} 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 parsecscale 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 H_{2} 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 H_{2} mass.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 2022
 DOI:
 10.3847/15384357/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
 EPrint:
 Published in ApJ (typos in the abstract fixed). Code for interpolating Lagrangian (particle) data onto an adaptive mesh (and auxiliary scripts for RADMC3D) available at https://github.com/huchiayu/ParticleGridMapper.jl