The Formation and Destruction of Molecular Clouds and Galactic Star Formation

TL;DR

This paper presents a comprehensive model of galactic star formation, highlighting molecular cloud formation through multiple compression events, their destruction by stellar radiation, and the evolution of cloud mass functions over time.

Contribution

It introduces a scenario where molecular clouds form in interacting shells on galactic scales and evaluates their formation, destruction, and star formation efficiency through simulations.

Findings

Molecular clouds form via multiple supersonic compression episodes.

Stellar FUV radiation effectively destroys magnetized molecular clouds.

Star formation efficiency within clouds is a few percent.

Abstract

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 estimate the ensemble-averaged growth rate of molecular clouds over a timescale larger than a million years. Next we perform radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar FUV radiation. We also investigate the resultant star formation efficiency within a cloud which amounts to a low value (a few percent) if we adopt the power-law exponent -2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds over a long timescale (>1Myr) and discuss the steady state exponent of the power-law slope in various environments.