The Single-Cloud Star Formation Relation

Abstract

One of the most important and well-established empirical results in astronomy is the so-called Kennicutt-Schmidt (KS) relation between the density of interstellar gas and the rate at which that gas forms stars. While a tight correlation between these quantities has long been measured at galactic scales, the difficulty of measuring star formation rates and gas densities over a large dynamic range at sub-galactic scales has thus far precluded a definitive determination of whether the same relationship holds within individual star-forming clouds. In this article we use a new, high-accuracy catalogue of young stellar objects from Spitzer combined with new, high-dynamic range maps of twelve nearby ($<$1.5 kpc) molecular clouds from Herschel to re-examine the KS relation within individual molecular clouds. We find a tight, linear correlation between clouds' star formation rate per unit area and their gas surface density normalised by the gas free-fall time. The measured relation extends over more than two orders of magnitude within each cloud, and is nearly identical in each of the twelve clouds, implying a constant star formation efficiency per free-fall time eff≈0.026$. The finding of a universal correlation within individual molecular clouds, including clouds that contain no massive stars or massive stellar feedback, favours models in which star formation is regulated by local processes such as turbulence or protostellar outflows, and disfavours models in which star formation is regulated primarily by galaxy properties or supernova feedback on galactic scales.