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

By S. Longmore, J. Bally, L. Testi, C. Purcell, A. Walsh, E. Bressert, M. Pestalozzi, Sergio Molinari, Juerg Ott, L. Cortese and 6 more. 2012

262 citations recorded by the source corpus.

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 disc 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 degrees several 10(3) cm(-3)) 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 must be included in the star formation prescription to predict the star formation rate in a given mass of molecular gas. We discuss which physical mechanisms might be responsible for suppressing star formation in the CMZ.

Subfields: Astronomy and Astrophysics, Fluid Flow and Transfer Processes

View Paper: https://www.semanticscholar.org/paper/74a44951106753b502c6266a7db00069a77268e8

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Citations in the corpus

Molecular gas kinematics within the central 250 pc of the Milky Way (2016, background): “Gas column and volume densities can be ∼ 2 orders of magnitude greater (Longmore et al. 2013a; Kruijssen et al. 2014; Rathborne et al. 2014b), velocity dispersions measured for a given physical size are larger (e.g. Bally et al. 1988; Shetty et al. 2012), and although there exists a significant…”
The ATLAS3D Project – XXVIII. Dynamically driven star formation suppression in early-type galaxies (2014, background): “A similar suppression of star formation may also be ongoing inthe central parts of our own Milky Way (Longmore et al. 2013), suggesting this may be a general process in spheroids and/or dense stellar environments.”
An uncertainty principle for star formation - I. Why galactic star formation relations break down below a certain spatial scale (2014, background): “This is due to the low SFR measured in the CMZ (Longmore et al. 2013), but even if the CMZ were forming stars at a higher rate, the effect of drift would still be more important than the incomplete sampling of star-forming regions (i.e. ∆xdrift > ∆xsamp).”
STAR FORMATION RELATIONS IN NEARBY MOLECULAR CLOUDS (2014, background): “However, the particular threshold that applies to nearby clouds may not apply in other regions, especially those with lower metallicity or much stronger radiation fields (cf. clouds in the CMZ, Longmore et al. 2013).”
Dense gas in the Galactic central molecular zone is warm and heated by turbulence (2015, background): “The Galactic CMZ has attracted a great deal of attention recently because it has a much higher ratio of dense gas mass to star formation tracers than the rest of the galaxy (Longmore et al. 2013b, 2012, 2013a; Kruijssen et al. 2014; Yusef-Zadeh et al. 2009; Immer et al. 2012).”
THE WISE CATALOG OF GALACTIC H ii REGIONS (2013, background): “Star formation is deficient in the Galactic center relative to the amount of molecular material available (Longmore et al. 2013, see also Beuther et al. (2012); Simpson et al. (2012)).”
Star formation rates and efficiencies in the Galactic Centre (2017, methodology): “…has been noted for several decades that despite harbouring this vast reservoir of dense gas, the CMZ appears to be underproducing stars with respect to nearby star-forming regions in the disc (e.g. Caswell et al. 1983; Guesten & Downes 1983; Taylor, Morris & Schulman 1993; Longmore et al. 2013a).”
What controls star formation in the central 500 pc of the Galaxy (2013, methodology): “Specifically, the CMZ contains∼ 80% of the NH3(1, 1) integrated intensity in the Galaxy (Longmore et al. 2013a), reflecting an overwhelming abundance of dense gas (n > a few × 103 cm−3).”
The Fermi Galactic Center GeV Excess and Implications for Dark Matter (2017, background): “The CMZ contains very dense molecular clouds that can host intensive star formation (e.g., Longmore et al. 2013) and, as a result, a significant rate of supernova explosions.”
The ATLASGAL survey: a catalog of dust condensations in the Galactic plane (2013, background): “Using various datasets other authors arrive at the same conclusion, that the Galactic center region has an exceptionally low SFR (Immer et al. 2012; Urquhart et al. 2013a; Longmore et al. 2013).”
The meaning and consequences of star formation criteria in galaxy models with resolved stellar feedback (2013, background): “…the central molecular zone is observed to have very high gas densities relative to the solar neighborhood and even many GMCs, but is not strongly self-gravitating, and so appears to have a SFR far lower than what would be predicted by a simple density threshold argument (see Longmore et al. 2013).”
A census of dense cores in the Aquila cloud complex: SPIRE/PACS observations from the Herschel Gould Belt survey (2015, background): “…since it does not seem to apply to the extreme environmental conditions of the central molecular zone near the Galactic center, for instance, where star formation is observed to be more inefficient above the same density threshold, by more than an order of magnitude (Longmore et al. 2013).”
The dynamical evolution of molecular clouds near the Galactic Centre - I. Orbital structure and evolutionary timeline (2014, methodology): “We show that the sequence of protocluster clouds identified by Longmore et al. (2013b) follows a coherent structure in position-velocity space.”
ATLASGAL — towards a complete sample of massive star forming clumps ⋆ (2014, methodology): “Furthermore, we used the Galactic distribution of the methanol masers to estimate the star formation efficiency (SFE) across the inner Galaxy and found it to be approximately three times lower in the Galactic Centre than in the disc (cf. Longmore et al. 2013).”