Variations in the Galactic star formation rate and density thresholds for star formation

TL;DR

This study shows that the current star formation rate in the Milky Way's Central Molecular Zone is much lower than predicted by existing models, suggesting additional factors like turbulence influence star formation.

Contribution

It demonstrates that existing density-based star formation prescriptions are insufficient in the CMZ, highlighting the need to include turbulence in models.

Findings

Star formation rate in the CMZ is an order of magnitude lower than predictions.

Dense molecular gas in the CMZ could form many clusters, but star formation is suppressed.

Turbulence amplitude must be considered alongside density in star formation models.

Abstract

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.