2018ApJ...858...16G

CO(J=1–0) line emission is a widely used observational tracer of molecular gas, rendering essential the X_CO factor, which is applied to convert CO luminosity to H₂ mass. We use numerical simulations to study how X_CO depends on numerical resolution, non-steady-state chemistry, physical environment, and observational beam size. Our study employs 3D magnetohydrodynamics (MHD) simulations of galactic disks with solar neighborhood conditions, where star formation and the three-phase interstellar medium (ISM) are self-consistently regulated by gravity and stellar feedback. Synthetic CO maps are obtained by postprocessing the MHD simulations with chemistry and radiation transfer. We find that CO is only an approximate tracer of H₂. On parsec scales, W_CO is more fundamentally a measure of mass-weighted volume density, rather than H₂ column density. Nevertheless, <X_CO >=(0.7-1.0)×10²⁰cm^–2K^–1km^–1s, which is consistent with observations and insensitive to the evolutionary ISM state or radiation field strength if steady-state chemistry is assumed. Due to non-steady-state chemistry, younger molecular clouds have slightly lower <X_CO>and flatter profiles of X_CO versus extinction than older ones. The CO-dark H₂ fraction is 26%-79%, anticorrelated with the average extinction. As the observational beam size increases from 1 to 100 pc, <X_CO>increases by a factor of ∼2. Under solar neighborhood conditions, <X_CO>in molecular clouds is converged at a numerical resolution of 2 pc. However, the total CO abundance and luminosity are not converged even at the numerical resolution of 1 pc. Our simulations successfully reproduce the observed variations of X_CO on parsec scales, as well as the dependence of X_CO on extinction and the CO excitation temperature.