There are no starless massive proto-clusters in the first quadrant of the Galaxy

TL;DR

This study searches for massive proto-clusters in the first Galactic quadrant, finding none without star formation activity, and concludes the starless phase of massive cluster formation is very short, less than 0.5 million years.

Contribution

It provides the first comprehensive search for starless massive proto-clusters in the first Galactic quadrant, establishing a short upper limit on their starless phase.

Findings

All identified proto-clusters are actively forming stars.

The Galactic massive cluster formation rate is about 5 per million years.

The starless phase of massive cluster formation is less than 0.5 million years.

Abstract

We search the lambda = 1.1 mm Bolocam Galactic Plane Survey for clumps containing sufficient mass to form ~10^4 M\odot star clusters. 18 candidate massive proto-clusters are identified in the first Galactic quadrant outside of the central kiloparsec. This sample is complete to clumps with mass M(clump) > 10^4 M_sun and radius r < 2.5 pc. The overall Galactic massive cluster formation rate is CFR(M_cluster > 10^4) ~ 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 t_starless < 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.