Effects of dust evolution on the abundances of CO and H2

Abstract

The CO-to-H2 conversion factor (XCO) is known to correlate with the metallicity (Z). The dust abundance, which is related to the metallicity, is responsible for this correlation through dust shielding of dissociating photons and H2 formation on dust surfaces. In this paper, we investigate how the relation between dust-to-gas ratio and metallicity (D-Z relation) affects the H2 and CO abundances (and XCO) of a 'molecular' cloud. For the D-Z relation, we adopt a dust evolution model developed in our previous work, which treats the evolution of not only dust abundance but also grain sizes in a galaxy. Shielding of dissociating photons and H2 formation on dust are solved consistently with the dust abundance and grain sizes. As a consequence, our models predict consistent metallicity dependence of XCO with observational data. Among various processes driving dust evolution, grain growth by accretion has the largest impact on the XCO-Z relation. The other processes also have some impacts on the XCO-Z relation, but their effects are minor compared with the scatter of the observational data at the metallicity range (Z ≳ 0.1 Z⊙) where CO could be detected. We also find that dust condensation in stellar ejecta has a dramatic impact on the H2 abundance at low metallicities (≲ 0.1 Z⊙), relevant for damped Lyman α systems and nearby dwarf galaxies, and that the grain size dependence of H2 formation rate is also important.