The formation and destruction of molecular clouds and galactic star formation An origin for the cloud mass function and star formation efficiency

We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with cooling, heating, and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First, we estimated the ensemble-averaged growth rate of molecular clouds on a timescale longer than a million years. Next, we performed radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar far-ultraviolet radiation. We also investigated the resulting star formation efficiency within a cloud, which amounts to a low value (a few percent) if we adopt the power-law exponent similar to- 2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds on a long timescale (>1 Myr) and discuss the steady state exponent of the power-law slope in various environments.