The Exciting Lives of Giant Molecular Clouds
  We present a detailed study of the evolution of GMCs in a galactic disc
simulation. We follow individual GMCs (defined in our simulations by a total
column density criterion), including their level of star formation, from their
formation to dispersal. We find the evolution of GMCs is highly complex. GMCs
often form from a combination of smaller clouds and ambient ISM, and similarly
disperse by splitting into a number of smaller clouds and ambient ISM. However
some clouds emerge as the result of the disruption of a more massive GMC,
rather than from the assembly of smaller clouds. Likewise in some cases, clouds
accrete onto more massive clouds rather than disperse. Because of the
difficulty of determining a precursor or successor of a given GMC, determining
GMC histories and lifetimes is highly non-trivial. Using a definition relating
to the continuous evolution of a cloud, we obtain lifetimes typically of 4-25
Myr for >10^5 M$_{\odot}$ GMCs, over which time the star formation efficiency
is about 1 %. We also relate the lifetime of GMCs to their crossing time. We
find that the crossing time is a reasonable measure of the actual lifetime of
the cloud, although there is considerable scatter. The scatter is found to be
unavoidable because of the complex and varied shapes and dynamics of the
clouds. We study cloud dispersal in detail and find both stellar feedback and
shear contribute to cloud disruption. We also demonstrate that GMCs do not
behave as ridge clouds, rather massive spiral arm GMCs evolve into smaller
clouds in inter-arm spurs.
