Abstract
ABSTRACT 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.
Highlights
Understanding the molecular gas content of galaxies is crucial for understanding the process of star formation (e.g., McKee & Ostriker 2007; Kennicutt & Evans 2012; Krumholz 2014)
When computing the Krumholz & Gnedin (2011) estimate, we assume the radiation field is a function of density and metallicity, which may differ from the inhomogeneous UV background computed in the FIRE-2 simulations that we are using in our chemical modelling
In this paper we have presented models of CO(1-0) emission in cosmological zoom simulations of Milky Way-mass galaxies which reproduce many of the observed properties of our Galaxy
Summary
Understanding the molecular gas content of galaxies is crucial for understanding the process of star formation (e.g., McKee & Ostriker 2007; Kennicutt & Evans 2012; Krumholz 2014). Some works have focused on modelling individual GMCs (e.g., Glover & Mac Low 2011; Shetty et al 2011a,b; Clark & Glover 2015; Penaloza et al 2018) This allows for high spatial resolution in the regions hosting the molecular gas, but this comes at the cost of having to neglect the impact of the larger scale galactic environment. Another approach is to model the emission coming from entire or parts of isolated galaxies (e.g., Duarte-Cabral et al 2015; Glover & Smith 2016; Richings & Schaye 2016; Gong et al 2018).
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