THE DISRUPTION OF GIANT MOLECULAR CLOUDS BY RADIATION PRESSURE & THE EFFICIENCY OF STAR FORMATION IN GALAXIES

Abstract

Star formation is slow in the sense that the gas consumption time is much longer than the dynamical time. It is also inefficient; star formation in local galaxies takes place in giant molecular clouds (GMCs), but the fraction of a GMC converted to stars is very small, GMC ∼ 5%. In 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. 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 H ii 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, a combination of three mechanisms—jets, H ii 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 disruption. We predict the presence of ∼10–20 clusters with masses ∼10 7 M ☉ in local ULIRGs such as Arp 220 and a similar number of clusters with M * ∼ 10 8 M ☉ in high redshift clump galaxies; submillimeter galaxies will have even more massive clusters. We find that GMC = π G Σ GMC c /(2( L / M * )) for GMCs that are optically thin to far-infrared radiation, where Σ GMC is the GMC gas surface density. The efficiency in optically thick systems continues to increase with Σ GMC , but more slowly, reaching ∼35% in the most luminous starbursts. The disruption of bubbles by radiation pressure stirs the interstellar medium (ISM) to velocities of ∼10 km s −1 in normal galaxies and to ∼100 km s −1 in ULIRGs like Arp 220, consistent with observations. Thus, radiation pressure may play a dominant dynamical role in the ISM of star-forming galaxies.