Variations in the Galactic star formation rate and density thresholds
  for star formation
  The conversion of gas into stars is a fundamental process in astrophysics and
cosmology. Stars are known to form from the gravitational collapse of dense
clumps in interstellar molecular clouds, and it has been proposed that the
resulting star formation rate is proportional to either the amount of mass
above a threshold gas surface density, or the gas volume density. These
star-formation prescriptions appear to hold in nearby molecular clouds in our
Milky Way Galaxy's disk as well as in distant galaxies where the star formation
rates are often much larger. The inner 500 pc of our Galaxy, the Central
Molecular Zone (CMZ), contains the largest concentration of dense, high-surface
density molecular gas in the Milky Way, providing an environment where the
validity of star-formation prescriptions can be tested. Here we show that by
several measures, the current star formation rate in the CMZ is an
order-of-magnitude lower than the rates predicted by the currently accepted
prescriptions. In particular, the region 1 deg < l < 3.5 deg, |b| < 0.5 deg
contains ~10^7 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. In addition to density, another property of
molecular clouds, such as the amplitude of turbulent motions, must be included
in the star-formation prescription to predict the star formation rate in a
given mass of molecular gas.
