The exciting lives of giant molecular clouds

We present a detailed study of the evolution of giant molecular clouds (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, and GMCs cannot be considered as isolated objects. GMCs often form from a combination of smaller clouds and ambient interstellar medium (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 on to 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-circle dot GMCs, over which time the star formation efficiency is about 1 per cent. 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 interarm spurs.