Observational Evidence of Dynamic Star Formation Rate in Milky Way Giant Molecular Clouds

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 ɛ and the star formation rate per freefall time ɛ _ff. Both ɛ and ɛ _ff range over 3-4 orders of magnitude. We find that 68.3% of the clouds fall within {σ }_{logɛ }=0.79+/- 0.22 {dex} and {σ }_{log{ɛ }_{ff}}=0.91+/- 0.22 {dex} about the median. Compared to these observed scatters, a simple model with a time-independent ɛ _ff that depends on the host GMC properties predicts {σ }_{log{ɛ }_{ff}}=0.12{--}0.24. Allowing for a time-variable ɛ _ff, 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.