THERE ARE NO STARLESS MASSIVE PROTO-CLUSTERS IN THE FIRST QUADRANT OF THE GALAXY
We search the λ = 1.1 mm Bolocam Galactic Plane Survey for clumps containing sufficient mass to form ∼104 M☉ star clusters. Eighteen candidate massive proto-clusters are identified in the first Galactic quadrant outside of the central kiloparsec. This sample is complete to clumps with mass Mclump > 104 M☉ and radius r ≲ 2.5 pc. The overall Galactic massive cluster formation rate is CFR(Mcluster > 104) ≲ 5 Myr−1, which is in agreement with the rates inferred from Galactic open clusters and M31 massive clusters. We find that all massive proto-clusters in the first quadrant are actively forming massive stars and place an upper limit of τstarless < 0.5 Myr on the lifetime of the starless phase of massive cluster formation. If massive clusters go through a starless phase with all of their mass in a single clump, the lifetime of this phase is very short.