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

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-ofmagnitude lower than the rates predicted by the currently accepted prescriptions. In particular, the region 1 • < l < 3.5 • , |b| < 0.5 • contains ∼ 10 7 M ⊙ 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.The rate at which gas is converted into stars has been measured in the disks of nearby galaxies. When averaged over hundreds of parsecs, the star-formation rate (SFR) was found to have a power-law dependence on the gas surface-density as described by the Schmidt-Kennicutt (SK) relations (Schmidt 1959;Kennicutt 1998;Kennicutt & Evans 2012). A linear relationship is found between SFR and gas surface-density above a local extinction threshold, AK ∼0.8 magnitudes at a near-infrared wavelength of 2.2 µm, corresponding to a gas column-density of ∼ 7.4 × 10 21

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…However, it is not so clear whether the same principles hold in the central molecularzone (CMZ)-a much more extreme environment. For instance, despite the high gas densities and the large amount of available gas, there is about an order of magnitude less active star formation in the CMZ than expected (Longmore et al 2013a;Johnston et al 2014;Kruijssen et al 2014). In order to test theories of star formation, our main aim here is to measure the amount and structure of the turbulence and to determine the magnetic field.…”

Conversion of gas into stars in the Galactic center

“…Indeed, the gas in the inner 100 pc of the Milky Way has been successfully modeled as a circumnuclear ring (Molinari et al 2011), requiring a low volume filling factor. These lines of evidence suggest that the average gas density in the CMZ is closer to 10 4 cm −3 (see Longmore et al 2013a for a detailed discussion). In which case, the gas in the CMZ and disk of the Milky Way can not fit on the same density-dependent star formation relations.…”

“…Yusef-Zadeh et al 2009, Immer et al 2012a, Longmore et al 2013a). These studies have reached different conclusions about whether the gas in the CMZ is consistent with the predictions of galactic-scale star formation relations, leading to some confusion.…”

Molecular gas kinematics of the CMZ: Great oaks from little acorns grow

“…In spite of this, the present-day star formation rate (∼ 0.05 M yr −1 ; e.g. Crocker 2012) is 1-2 orders of magnitude lower than one might naively expect when only considering the reservoir of dense ( 10 3 cm −3 ) gas (Longmore et al 2013a;Kruijssen et al 2014).…”