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