Northern Galactic Molecular Cloud Clumps in Hi-GAL: Clump and Star Formation within Clouds

Abstract

Abstract We investigate how the properties of Galactic giant molecular clouds (GMCs) and their denser substructures (clumps) correlate with the local star formation rate (SFR). We trace clouds using the 12CO(3−2) transition, as observed by the CO High Resolution Survey. We identify their constituent clumps using thermal dust emission, as observed by the Herschel infrared GALactic plane survey. We estimate SFRs in these clouds using 70 μm emission. In total, we match 3674 clumps to 473 clouds in position–position–velocity space spanning the Galactic longitude range 10° < ℓ < 56°. We find that more massive clouds produce more clumps and more massive clumps. These clumps have average number densities an order of magnitude greater than their host clouds. We find a mean clump mass fraction of . This mass fraction weakly varies with mass and mass surface density of clouds, and shows no clear dependence on the virial parameter and line width of the clouds. The average clump mass fraction is only weakly dependent upon Galactocentric radius. Although the scatter in our measured properties is significant, the SFR for clouds is independent of clump mass fraction. However, there is a positive correlation between the depletion times for clouds and clump mass fraction. We find a star formation efficiency per freefall time of ϵ ff = 0.15% for GMCs but ϵ ff = 0.37% for clumps.