The X CO Conversion Factor from Galactic Multiphase ISM Simulations

Munan Gong, Eve Charis Ostriker, Chang Goo Kim

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52 Scopus citations


CO(J = 1-0) line emission is a widely used observational tracer of molecular gas, rendering essential the XCO factor, which is applied to convert CO luminosity to H2 mass. We use numerical simulations to study how XCO 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, WCO is more fundamentally a measure of mass-weighted volume density, rather than H2 column density. Nevertheless, 〈XCO= (0.7-1.0) × 1020 cm-2 K-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 〈XCO〉 and flatter profiles of XCO 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, 〈XMCO〉 increases by a factor of ∼2. Under solar neighborhood conditions, 〈XCO〉 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 XCO on parsec scales, as well as the dependence of XCO on extinction and the CO excitation temperature.

Original languageEnglish (US)
Article number16
JournalAstrophysical Journal
Issue number1
StatePublished - May 1 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: abundances
  • ISM: clouds
  • ISM: molecules
  • galaxies: ISM
  • molecular processes


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