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We present models of CO(1-0) emission from Milky Way-mass galaxies atredshift zero in the FIRE-2 cosmological zoom-in simulations. We calculate themolecular abundances by post-processing the simulations with an equilibriumchemistry solver while accounting for the effects of local sources, anddetermine the emergent CO(1-0) emission using a line radiative transfer code.We find that the results depend strongly on the shielding length assumed, whichin our models sets the attenuation of the incident UV radiation field. At theresolution of these simulations, commonly used choices for the shieldinglength, such as the Jeans length, result in CO abundances that are too high ata given H$_2$ abundance. We find that a model with a distribution of shieldinglengths, which has a median shielding length of $\sim 3$ pc in cold gas ($T <300$ K) for both CO and H$_{2}$, is able to reproduce both the observed CO(1-0)luminosity and inferred CO-to-H$_{2}$ conversion factor at a given starformation rate compared with observations. We suggest that this short shieldinglength can be thought of as a subgrid model which controls the amount ofradiation that penetrates giant molecular clouds.

Reproducing the CO-to-H$_2$ conversion factor in cosmological simulations of Milky Way-mass galaxies