Observational evidence of dynamic star formation rate in Milky Way giant molecular clouds.
LEE E.J., MIVILLE-DESCHENES M.-A. and MURRAY N.W.
Abstract (from CDS):
Star formation on galactic scales is known to be a slow process, but whether it is slow on smaller scales is uncertain. We cross-correlate 5469 giant molecular clouds (GMCs) from a new all-sky catalog with 256 star-forming complexes (SFCs) to build a sample of 191 SFC-GMC complexes-collections of multiple clouds each matched to 191 SFCs. The total mass in stars harbored by these clouds is inferred from WMAP free-free fluxes. We measure the GMC mass, the virial parameter, the star formation efficiency ε and the star formation rate per freefall time εff. Both ε and εff range over 3-4 orders of magnitude. We find that 68.3% of the clouds fall within σlogε=0.79±0.22 dex and σlogεff_=0.91±0.22 dex about the median. Compared to these observed scatters, a simple model with a time-independent εff that depends on the host GMC properties predicts σlogεff_=0.12–0.24. Allowing for a time-variable εff, we can recover the large dispersion in the rate of star formation. This strongly suggests that star formation in the Milky Way is a dynamic process on GMC scales. We also show that the surface star formation rate profile of the Milky Way correlates well with the molecular gas surface density profile.