The Disruption of Giant Molecular Clouds by Radiation Pressure & the Efficiency of Star Formation in Galaxies

Star formation is slow, in the sense that the gas consumption time is much longer than the dynamical time. It is also inefficient; essentially all star formation in local galaxies takes place in giant molecular clouds (GMCs), but the fraction of a GMC converted to stars is very small, ∼ 5%. While there is some disagreement over the lifespan of GMCs, there is a consensus that it is no more than a few cloud dynamical times. In the most luminous starbursts, the GMC lifetime is shorter than the main sequence lifetime of even the most massive stars, so that supernovae can play no role in GMC disruption; another feedback mechanism must dominate. We investigate the disruption of GMCs across a wide range of galaxies, from normal spirals to the densest starbursts; we take into account the effects of HII gas pressure, shocked stellar winds, protostellar jets, and radiation pressure produced by the absorption and scattering of starlight on dust grains. In the Milky Way, we find that a combination of three mechanisms — jets, HII gas pressure, and radiation pressure — disrupts the clouds. In more rapidly star forming galaxies such as “clump” galaxies at high-redshift, ultra-luminous infrared galaxies (ULIRGs) and submillimeter galaxies, radiation pressure dominates natal cloud distribution. We predict the presence of ∼ 10 − 20 clusters with masses ∼ 10M⊙ in local ULIRGs such as Arp 220 and a similar number of clusters with M∗ ∼ 10M⊙ in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters. We find that the mass fraction of a GMC that ends up in stars is an increasing function of the gas surface density of a galaxy, reaching ∼ 35% in the most luminous starbursts. Furthermore, the disruption of bubbles by radiation pressure stirs the interstellar medium to velocities of ∼ 10km s−1 in normal galaxies and to ∼ 100km s−1 in ULIRGs like Arp 220, consistent with observations. Thus, radiation pressure may play a dominant role in the ISM of star-forming galaxies. Subject headings: Galaxies: star clusters, formation, general, starburst — HII regions — ISM: clouds, bubbles — stars: formation