The life-cycle of molecular clouds in disk galaxies

Molecular clouds are cold, dense, and turbulent filamentary structures that condense out of the multi-phase interstellar medium. They are also the sites of star formation. The minority of new-born stars is massive, but these stars are particularly important for the fate of their parental molecular clouds. I will present results from high- performance, three-dimensional simulations that show the formation and dispersal of molecular clouds within representative pieces of disk galaxies. Apart from stellar feedback and self-gravity, we employ an accurate description of gas heating and cooling as well as a small chemical network including molecule formation and self- shielding. Gravitational collapse is compensated by stellar feedback, leading to the establishment of a dynamical equilibrium of the interstellar medium within the disk. I will discuss results for disks at different gas surface densities which e.g. demonstrate that the molecular gas mass fraction increases with gas surface density. Moreover, I will show that outflows generated by supernovae that explode within the parental molecular clouds may contain a significant fraction of diffuse molecular hydrogen that is not well traced by CO. These simulations will bring forth a modern paradigm for the full life cycle of molecular clouds with important implications for galaxy evolution.