What can simulated molecular clouds tell us about real molecular clouds?
  We study the properties of giant molecular clouds (GMCs) from a smoothed
particle hydrodynamics simulation of a portion of a spiral galaxy, modelled at
high resolution, with robust representations of the physics of the interstellar
medium. We examine the global molecular gas content of clouds, and investigate
the effect of using CO or H2 densities to define the GMCs. We find that CO can
reliably trace the high-density H2 gas, but misses less dense H2 clouds. We
also investigate the effect of using 3D CO densities versus CO emission with an
observer's perspective, and find that CO-emission clouds trace well the peaks
of the actual GMCs in 3D, but can miss the lower density molecular gas between
density peaks which is often CO-dark. Thus, the CO emission typically traces
smaller clouds within larger GMC complexes. We also investigate the effect of
the galactic environment (in particular the presence of spiral arms), on the
distribution of GMC properties, and we find that the mean properties are
similar between arm and inter-arm clouds, but the tails of some distributions
are indicative of intrinsic differences in the environment. We find highly
filamentary clouds (similar to the giant molecular filaments of our Galaxy)
exclusively in the inter-arm region, formed by galactic shear. We also find
that the most massive GMC complexes are located in the arm, and that as a
consequence of more frequent cloud interactions/mergers in the arm, arm clouds
are more sub-structured and have higher velocity dispersions than inter-arm
clouds.
