Star formation in 'the Brick': ALMA reveals an active proto-cluster in the Galactic centre cloud G0.253+0.016

G0.253+0.016, also known as 'the Brick', is one of the most massive and dense molecular clouds in the Milky Way's Central Molecular Zone (inner few hundred parsecs). Previous observations have detected tentative signs of active star formation, most notably a single water maser associated with a dust continuum source. In this work we present the highest angular resolution ALMA Band 6 observations of this cloud to date (0.13 arcseconds, 1000 AU). We detect a population of eighteen continuum sources (median mass ~ 2 M), nine of which are driving bi-polar molecular outflows as seen via SiO (5-4) emission (see image below). This constitutes the first unambiguous detection of active star formation in this extreme molecular cloud.

Despite the high density of G0.253+0.016, we find no evidence for high-mass protostars in our ALMA field. The observed sources are instead consistent with a cluster of low-to-intermediate-mass protostars. However, the measured outflow properties are consistent with those expected for intermediate-to-high-mass star formation. We conclude that the sources are young and rapidly accreting, and may potentially form intermediate and high-mass stars in the future. The masses and projected spatial distribution of the cores are generally consistent with thermal fragmentation, suggesting that the large-scale turbulence and strong magnetic field in the cloud do not dominate on these scales, and that star formation on the scale of individual protostars is similar to that in Galactic disc environments.

D. L. Walker et al., 2021, MNRAS, 503, 77 (view paper on arXiv)

Star formation in a high pressure environment: an SMA view of the Galactic Centre dust ridge

The star formation rate in the Central Molecular Zone (CMZ) is an order of magnitude lower than predicted according to star formation relations that have been calibrated in the disc of our own and nearby galaxies. Understanding how and why star formation appears to be different in this region is crucial if we are to understand the environmental dependence of the star formation process. In this work, we present observations of some of the most massive and dense molecular clouds in the CMZ using the Submillimeter Array. These observations were taken as part of the larger CMZoom survey, and observed the clouds at an angular resolution of 3 arcseconds (0.1 parsecs) at 230 GHz (1.3 mm).

We detect a population of bright, compact sources in the dust continuum, ranging in mass from ~ 50 - 2150 M within radii of 0.1 - 0.25 pc. All appear to be young, meaning that they are prime candidates for representing the initial conditions of high-mass stars and sub-clusters. At least two of the sources are confirmed to be high-mass star-forming regions. We compare our sample with similar sources in the comparatively less extreme environment of the Galactic disc, and find that they are broadly similar despite being subjected to external pressures that may be up to three orders of magnitude greater. The fact that > 80% of these sources do not show any signs of star-forming activity in such a high-pressure environment leads us to conclude that this is evidence for an increased critical density threshold for star formation in the CMZ.

D. L. Walker et al., 2018, MNRAS, 474, 2373 (view paper on arXiv)

Young massive star cluster formation throughout the Milky Way

Young massive clusters (YMCs) have central stellar mass surface densities exceeding 104 M pc-2. It is thought that they are the present-day analogues of the old (Gyr) globular clusters that formed in the early Universe. It is currently unknown whether the stars in these YMCs formed at such high (proto)stellar densities. To investigate this, we conducted a series of studies in which we compiled data for the known Galactic YMCs, and compared their stellar distributions with the distribution of gas in all of the known candidate YMC-precursor molecular clouds.

We find that the gas in the YMC progenitor clouds is distributed very differently than the stars in the YMCs. The mass surface density profiles of the gas clouds are generally much shallower than the stellar mass surface density profiles of the YMCs, which are characterised by prominent dense core regions with radii ~ 0.1 pc, followed by a power-law tail. On the scale of YMC core radii, we find that there are no known clouds in the Milky Way with significantly more mass in their central regions when compared to Galactic YMCs. Additionally, we find that models in which stars form from very dense initial conditions require surface densities that are higher than those observed in the known candidate YMC progenitor clouds.

Our results show that the quiescent, less evolved clouds contain less mass in their central regions than in the highly star-forming clouds. This suggests an evolutionary trend in which clouds continue to accumulate mass towards their centres after the onset of star formation. We therefore conclude that a 'conveyor-belt' scenario for YMC formation is consistent with the current sample of Galactic YMCs and their progenitor clouds.

D. L. Walker et al., 2016, MNRAS, 457, 4536 (view paper on arXiv)

D. L. Walker et al., 2015, MNRAS, 449, 715 (view paper on arXiv)