Young massive star cluster formation in the Galactic Centre is driven by global gravitational collapse of high-mass molecular clouds

Abstract

Young massive clusters (YMCs) are the most compact, high-mass stellar systems still forming at the present day. The precursor clouds to such systems are, however, rare due to their large initial gas mass reservoirs and rapid dispersal timescales due to stellar feedback. Nonetheless, unlike their high-z counterparts, these precursors are resolvable down to the sites of individually forming stars, and hence represent the ideal environments in which to test the current theories of star and cluster formation. Using high angular resolution (1 / 0.05pc) and sensitivity ALMA observations of two YMC progenitor clouds in the Galactic Centre, we have identified a suite of molecular line transitions -e.g. c-C 3 H 2 (7 − 6) -that are believed to be optically thin, and reliably trace the gas structure in the highest density gas on star-forming core scales. We conduct a virial analysis of the identified core and proto-cluster regions, and show that half of the cores (5/10) and both proto-clusters are unstable to gravitational collapse. This is the first kinematic evidence of global gravitational collapse in YMC precursor clouds at such an early evolutionary stage. The implications are that if these clouds are to form YMCs, then they likely do so via the "conveyor-belt" mode, whereby stars continually form within dispersed dense gas cores as the cloud undergoes global gravitational collapse. The concurrent contraction of both the cluster-scale gas and embedded (proto)stars ultimately leads to the high (proto)stellar density in YMCs.

Which feedback mechanisms dominate in the high-pressure environment of the central molecular zone?

“…We also highlight the quiescent precursor molecular clouds on the "dust-ridge" (e.g. Lis et al 1994;Longmore et al 2012Longmore et al , 2013aWalker et al 2015Walker et al , 2016Walker et al , 2018; Barnes et al 2019;Henshaw et al 2019;Battersby et al 2020). This dust-ridge is thought to be connected to the ionised ridge via a ring of material surrounding the Galactic Centre found in simulations (e.g.…”

Star Clusters Near and Far

“…New instruments like large integral field units (IFUs), such as the Multi Object Spectrographic Explorer (MUSE, Bacon et al 2010) mounted at the VLT, the Gaia satellite (Prusti et al 2016;Brown et al 2018), and long baseline photometric observations allow us, for the first time, to study the detailed 3D dynamics of the majority of stars in these resolved star clusters, including the dynamics of the gas (e.g., Kamann et al 2013;McLeod et al 2015;Zeidler et al 2018;Lennon et al 2018;Wright and Mamajek 2018;Ward and Kruijssen 2018;Ward et al 2019;Getman et al 2019;Zari et al 2019). This provides insights into the star cluster formation modes: Do star clusters form hierarchically, following the structure of the giant molecular cloud (GMC) (e.g., Kruijssen et al 2012b;Parker et al 2014;Longmore et al 2014;Walker et al 2015Walker et al , 2016Barnes et al 2019;Ward et al 2019), or do they form in monolithic, central starburst-like events (e.g., Lada et al 1984;Bastian and Goodwin 2006;Banerjee and Kroupa 2015)? Future missions and telescopes, such as JWST, the E-ELT, or the Thirty Meter Telescope (TMT), will provide the necessary angular resolution and wavelength ranges to further investigate the low-mass end of the initial mass function (IMF) 1 and the embedded objects in the surrounding HII regions.…”

Making top-heavy IMFs from canonical IMFs near the Galactic Centre

“…So it is unclear if local regions are a good analogue to star formation in the GC (e.g. Barnes et al 2019). But we would argue that it is as reasonable a starting point as any.…”