A general model for the CO-H2 conversion factor in galaxies with applications to the star formation law

Abstract

The most common means of converting an observed CO line intensity into a\nmolecular gas mass requires the use of a conversion factor (Xco). While in the\nMilky Way this quantity does not appear to vary significantly, there is good\nreason to believe that Xco will depend on the larger-scale galactic\nenvironment. Utilising numerical models, we investigate how varying\nmetallicities, gas temperatures and velocity dispersions in galaxies impact the\nway CO line emission traces the underlying H2 gas mass, and under what\ncircumstances Xco may differ from the Galactic mean value. We find that, due to\nthe combined effects of increased gas temperature and velocity dispersion, Xco\nis depressed below the Galactic mean in high surface density environments such\nas ULIRGs. In contrast, in low metallicity environments, Xco tends to be higher\nthan in the Milky Way, due to photodissociation of CO in metal-poor clouds. At\nhigher redshifts, gas-rich discs may have gravitationally unstable clumps which\nare warm (due to increased star formation) and have elevated velocity\ndispersions. These discs tend to have Xco values ranging between present-epoch\ngas-rich mergers and quiescent discs at low-z. This model shows that on\naverage, mergers do have lower Xco values than disc galaxies, though there is\nsignificant overlap. Xco varies smoothly with the local conditions within a\ngalaxy, and is not a function of global galaxy morphology. We combine our\nresults to provide a general fitting formula for Xco as a function of CO line\nintensity and metallicity. We show that replacing the traditional approach of\nusing one constant Xco for starbursts and another for discs with our best-fit\nfunction produces star formation laws that are continuous rather than bimodal,\nand that have significantly reduced scatter.\n