The CO universe: modelling CO emission and H2 abundance in cosmological galaxy formation simulations

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ABSTRACT We devise a physical model of formation and distribution of molecular gas clouds (MGCs) in galaxies. We use the model to predict the intensities of rotational transition lines of carbon monoxide (CO) and the molecular hydrogen (H2) abundance. Using the outputs of IllustrisTNG cosmological simulations, we populate MGCs of unresolved sizes in individual simulated galaxies, where the effect of the interstellar radiation field with dust attenuation is also taken into account. We then use the publicly available code despotic (Derive the Energetics and SPectra of Optically Thick Interstellar Clouds) to compute the CO line luminosities and H2 densities without assuming the CO-to-H2 conversion factor (αCO). Our method allows us to study the spatial and kinematic structures traced by CO(1–0) and higher transition lines. We compare the CO luminosities and H2 masses with recent observations of galaxies at low and high redshifts. Our model reproduces well the observed CO–luminosity function and the estimated H2 mass in the local UniverseAbout 10 per cent of molecules in the Universe reside in dwarf galaxies with stellar masses lower than 109 M⊙, but the galaxies are generally ‘CO-dark’ and have typically high αCO. Our model predicts generally lower CO line luminosities than observations at redshifts z ≳ 1–2. We argue that the difference can be explained by the highly turbulent structure suggested for the high-redshift star-forming galaxies.