Star formation rates and efficiencies in the Galactic Centre

Abstract

The inner few hundred parsecs of the Milky Way harbours gas densities, pressures, velocity dispersions, an interstellar radiation field and a cosmic ray ionisation rate orders of magnitude higher than the disc; akin to the environment found in star-forming galaxies at high-redshift. Previous studies have shown that this region is forming stars at a rate per unit mass of dense gas which is at least an order of magnitude lower than in the disc, potentially violating theoretical predictions. We show that all observational star formation rate diagnostics -both direct counting of young stellar objects and integrated light measurements -are in agreement within a factor two, hence the low star formation rate is not the result of the systematic uncertainties that affect any one method. As these methods trace the star formation over different timescales, from 0.1 − 5 Myr, we conclude that the star formation rate has been constant to within a factor of a few within this time period. We investigate the progression of star formation within gravitationally bound clouds on ∼ parsec scales and find 1 − 4 per cent of the cloud masses are converted into stars per free-fall time, consistent with a subset of the considered "volumetric" star formation models. However, discriminating between these models is obstructed by the current uncertainties on the input observables and, most importantly and urgently, by their dependence on ill-constrained free parameters. The lack of empirical constraints on these parameters therefore represents a key challenge in the further verification or falsification of current star formation theories.

Cloud-scale ISM Structure and Star Formation in M51

“…It also matches the apparent requirements for turbulent models to match observations of dense gas, IR, and CO in nearby galaxies (García-Burillo et al 2012;Usero et al 2015). However, our inferred ǫ ff is much lower than values measured for the nearest molecular clouds by Evans et al (2014), Murray (2011), or Lee et al (2016 (see also Lada et al 2010Lada et al , 2012, as well as for molecular clouds orbiting the Galactic Center by Barnes et al (2017). It is also much lower than the values commonly adopted in analytic theories and numerical simulations (e.g., see Krumholz et al 2012;Agertz & Kravtsov 2015, among many others).…”

The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone

“…Barnes et al (2017) extend the SF sequence downstream from SgrB2 by finding evidence of a gradual increase in stellar feedback activity as measured from the size of the hot gas bubbles driven by young massive stars. The Arches and Quintuplet clusters are found to be consistent in kinematics and age to have formed by tidally triggered cloud collapse and thus seem to fit in the GC star formation model (K15).…”

The turbulent formation of stars

“…3 In particular, the theory of turbulence-regulated star formation predicts a rate of only 0.01 solar mass per year in the so-called Brick cloud near the galactic center, a rate that is indeed observed. 4 On larger, galaxy scales, shear can also contribute to reducing and regulating star formation.…”