Chemical evolution of giant molecular clouds in simulations of galaxies
  We present an analysis of Giant Molecular Clouds (GMCs) within hydrodynamic
simulations of isolated, low-mass (M* ~ 10^9 M_sol) 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 H2 and CO compared to chemical equilibrium,
by factors of ~3 and 1-2 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 - A_v 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.
