Molecular gas in the inner 500pc of the MilkyWay: violating star formation relations and on the verge of forming extreme stellar clusters

TL;DR

This study reveals that the inner 500pc of the Milky Way has a much lower star formation rate than expected given its dense molecular gas, challenging existing universal star formation relations and identifying a potential progenitor of a massive stellar cluster.

Contribution

It provides new observational evidence that the star formation rate in the Galactic center deviates from universal relations and identifies a unique molecular cloud likely to form an extreme stellar cluster.

Findings

Star formation rate in the inner 500pc is at least an order of magnitude lower than in the disk.

A dense, massive molecular cloud with minimal star formation may be a progenitor of a massive stellar cluster.

The observed molecular cloud is unique in the Galaxy, crucial for testing cluster formation models.

Abstract

With the HOPS, MALT90 and HiGAL Galactic plane surveys we are mapping a significant fraction of the dense, star-forming, molecular gas in the Galaxy. I present results from two projects based on this combined dataset, namely, (i) looking for variations in the star formation (SF) rate across the Galaxy as a function of environment, and (ii) searching for molecular cloud progenitors of the most extreme (massive and dense) stellar clusters. We find the SF rate per unit mass of dense gas in the inner 500pc of the Galaxy is at least an order of magnitude lower than that in the disk, directly challenging the predictions of proposed universal column/volume density relations. In particular, the region 1 degrees < l < 3.5 degrees, |b| < 0.5 degrees contains ~1E7 Msun of dense molecular gas -- enough to form 1000 Orion-like clusters -- but the present-day star formation rate within this gas is only equivalent to that in Orion. I present follow up studies of one molecular cloud we have studied as part of project (ii) which also lies in the inner 500 pc of the Galaxy and is clearly extreme compared to the rest of the Galactic population. With a mass of 1E5 Msun,a radius of only ~3pc and almost no signs of star formation it appears to be the progenitor of an Arches-like stellar cluster. Despite detailed observational followup searches, this object still appears to be unique in the Galaxy, making it extremely important for testing massive cluster formation models.