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; 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.