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

Abstract

We investigate how the properties of Galactic giant molecular clouds (GMCs) and their denser substructures (clumps) correlate with the local star formation rate. We trace clouds using the $^{12}$CO(3-2) transition, as observed by the CO High Resolution Survey (COHRS). We identify their constituent clumps using thermal dust emission, as observed by the Herschel infrared GALactic plane survey (Hi-GAL). We estimate star formation rates in these clouds using 70 $\mu$m emission. In total, we match 3,674 clumps to 473 clouds in position-position-velocity space spanning the Galactic longitude range $10^\circ<\ell<56^\circ$. 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 $0.20^{+0.13}_{-0.10}$. 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 star formation rate 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 free fall time of $\epsilon_{\mathrm{ff}}=0.15\%$ for GMCs but $\epsilon_{\mathrm{ff}}=0.37\%$ for clumps.