2016MNRAS.460.2297R

We present an analysis of giant molecular clouds (GMCs) within hydrodynamic simulations of isolated, low-mass (M_* ∼ 10^9 M_☉) disc galaxies. We study the evolution of molecular abundances and the implications for CO emission and the X_CO conversion factor in individual clouds. We define clouds either as regions above a density threshold n_H, min=10   {cm}^{-3}, or using an observationally motivated CO intensity threshold of 0.25   {K}   {km}   {s}^{-1}. Our simulations include a non-equilibrium chemical model with 157 species, including 20 molecules. We also investigate the effects of resolution and pressure floors (i.e. Jeans limiters). We find cloud lifetimes up to ≃ 40 Myr, with a median of 13 Myr, in agreement with observations. At one-tenth solar metallicity, young clouds ( <= 10-15 Myr) are underabundant in H_2 and CO compared to chemical equilibrium, by factors of ≃3 and one to two orders of magnitude, respectively. At solar metallicity, GMCs reach chemical equilibrium faster (within ≃ 1 Myr). We also compute CO emission from individual clouds. The mean CO intensity, I_CO, is strongly suppressed at low dust extinction, A_v, and possibly saturates towards high A_v, in agreement with observations. The I_CO–Av relation shifts towards higher A_v for higher metallicities and, to a lesser extent, for stronger UV radiation. At one-tenth solar metallicity, CO emission is weaker in young clouds ( <= 10-15 Myr), consistent with the underabundance of CO. Consequently, X_CO decreases by an order of magnitude from 0 to 15 Myr, albeit with a large scatter.