Reproducing the CO-to-H2conversion factor in cosmological simulations of Milky-Way-mass galaxies

Laura C. Keating*, Alexander J. Richings, Norman Murray, Claude André Faucher-Giguère, Philip F. Hopkins, Andrew Wetzel, Dušan Kereš, Samantha Benincasa, Robert Feldmann, Sarah Loebman, Matthew E. Orr

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


We present models of CO(1-0) emission from Milky-Way-mass galaxies at redshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate the molecular abundances by post-processing the simulations with an equilibrium chemistry solver while accounting for the effects of local sources, and determine the emergent CO(1-0) emission using a line radiative transfer code. We find that the results depend strongly on the shielding length assumed, which, in our models, sets the attenuation of the incident UV radiation field. At the resolution of these simulations, commonly used choices for the shielding length, such as the Jeans length, result in CO abundances that are too high at a given H2 abundance. We find that a model with a distribution of shielding lengths, which has a median shielding length of ~3 pc in cold gas (T < 300 K) for both CO and H2, is able to reproduce both the observed CO(1-0) luminosity and inferred CO-to-H2 conversion factor at a given star formation rate compared with observations. We suggest that this short shielding length can be thought of as a subgrid model, which controls the amount of radiation that penetrates giant molecular clouds.

Original languageEnglish (US)
Pages (from-to)837-850
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Nov 1 2020


  • Galaxies: ISM
  • Galaxies: evolution
  • ISM: molecules
  • Methods: numerical

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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