The ATLASGAL survey: The sample of young massive cluster progenitors

Abstract

Context. The progenitors of high-mass stars and clusters are still challenging to recognise. Only unbiased surveys, sensitive to compact regions of high dust column density, can unambiguously reveal such a small population of particularly massive and cold clumps. Aims. Here we use the ATLASGAL survey to identify a sample of candidate progenitors of massive clusters in the inner Galaxy. Methods. We characterise a flux limited sample of compact sources selected from the ATLASGAL survey. Sensitive mid-infrared data at 21−24 µm from the WISE and MIPSGAL surveys were explored to search for embedded objects, and complementary spectroscopic data were used to investigate their stability and their star formation activity. Results. We identify an unbiased sample of infrared-quiet massive clumps in the Galaxy that potentially represent the earliest stages of massive cluster formation. An important fraction of this sample consists of sources that have not been studied in detail before. We first find that clumps hosting more evolved embedded objects and infrared-quiet clumps exhibit similar physical properties in terms of mass and size, suggesting that the sources are not only capable of forming high-mass stars, but likely also follow a single evolutionary track leading to the formation of massive clusters. The majority of the clumps are likely not in virial-equilibrium, suggesting collapse on the clump scale. Conclusions. We identify the precursors of the most massive clusters in the Galaxy within our completeness limit, and argue that these objects are undergoing large-scale collapse. This is in line with the low number of infrared-quiet massive clumps and earlier findings that star formation, in particular for high-mass objects is a fast, dynamic process. We propose a scenario in which massive clumps start to fragment and collapse before their final mass is accumulated indicating that strong self-gravity and global collapse is needed to build up rich clusters and the most massive stars.

ALMA survey of massive cluster progenitors from ATLASGAL

“…To estimate the mass, we assume optically thin dust emission and use the same formula as in Csengeri et al (2017) as follows:…”

“…While on the ∼0.3 pc scales of clumps Csengeri et al (2017) adopt T d = 18 K, on smaller scales of cores heating due to the embedded protostar may result in elevated dust temperatures that are poorly constrained. Following the model of Goldreich & Kwan (1974), we estimate T d = 15−38 K for the luminosity range of 10 2 −10 4 L at a typical radius of half the deconvolved FWHM size of 0.025 pc.…”

“…Based on a flux limited sample of the 870 µm APEX Telescope LArge Survey of the GAlaxy (ATLASGAL; Schuller et al 2009;Csengeri et al 2014), Csengeri et al (2017) identified the complete sample of massive infrared quiet clumps with the highest Full Table A.1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/600/L10 peak surface density (Σ cl ≥ 0.5 g cm −2 ) 1 and low bolometric luminosity, L bol < 10 4 L , corresponding to the ZAMS luminosity of a late O-type star. Their large mass reservoir and low luminosity suggest that infrared quiet massive clumps correspond to the early evolutionary phase; some of them already exhibit signs of ongoing (high-mass) star formation, such as EGOs and Class II methanol masers.…”

“…Here we present the sample of 35 infrared quiet massive clumps located within d ≤ 4.5 kpc, which could be conveniently grouped on the sky as targets for ALMA. They cover 70% of all the most massive and nearby infrared quiet clumps from Csengeri et al (2017) and are thus a representative selection of a homogenous sample of early phase massive clumps in the inner Galaxy.…”

“…Large samples have only recently been identified based on large area surveys (e.g. Butler & Tan 2012;Tackenberg et al 2012;Traficante et al 2015;Svoboda et al 2016;Csengeri et al 2017), which show that the early evolutionary stages are short lived (e.g. Motte et al 2007;Csengeri et al 2014), as star formation proceeds rapidly.…”