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