The XCO Conversion Factor from Galactic Multiphase ISM Simulations

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 H-2 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 H-2. On parsec scales, W-CO is more fundamentally a measure of mass-weighted volume density, rather than H-2 column density. Nevertheless, < XCO > = (0.7-1.0) x 10(20) cm(-2) K-1 km(-1) s, 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 H-2 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 similar to 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.