Enigmatic cloud near the Galactic Centre may be richer in young stars than previously thought

A group of astronomers led by the Max Planck Institute for Astronomy found evidence for a young stellar cluster hidden inside a cloud known as “the Brick”. This cloud near the Galactic Centre so far appeared unusually quiescent regarding star formation. The new finding follows from an arc-shaped substructure whose properties are consistent with an expanding shell. The authors link it to a bubble of hot gas produced by the stellar wind of a young massive star. Since massive stars rarely form in isolation, the bubble could indicate the presence of a young stellar cluster, equivalent to several hundred solar masses.

Stars form inside dense pockets of clouds made of gas and dust. Generally, whenever a cloud is dense enough, stars will eventually form. However, this rule of thumb does not appear to hold entirely for the region around the Milky Way centre. The Central Molecular Zone (CMZ), a gas complex with a diameter between 1000 and 2000 light-years around the Galactic Centre, contains some of the densest and most massive gas clouds known in the Milky Way. However, apart from a few extraordinarily massive stellar clusters, many of those clouds show surprisingly little evidence of widespread star formation activity.

To explore this apparent contradiction, a group of astronomers led by Jonathan Henshaw from the Max Planck Institute for Astronomy (MPIA) in Heidelberg, Germany, investigated one of the most enigmatic clouds in the CMZ, called “the Brick”. It is well known for its high density and mass, equivalent to about 100,000 suns. Still, it appears to produce comparatively few stars.

“When investigating the gas motion of the Brick, we find one component that stands out,” says Jonathan Henshaw. He is the lead author of the underlying article published in the Monthly Notices of the Royal Astronomical Society. “That sub-structure, confined to a narrow range of velocities, resembles a crescent-shaped arc,” Henshaw adds.

Such arcs have been detected in regions of high-mass star formation. They can represent the material swept up by an expanding shell. High-mass stars deliver energy and momentum to their surroundings, which act as the driving force of the expansion. Based on this assumption, the research group derived a shell diameter of 8.5 light-years and an expansion velocity of approximately 5 kilometres per second. By tracing this motion back to its origin, the astronomers dated the beginning of the expansion back to a few hundred thousand years ago. This is just a wink of an eye, considering the timescales typical of cosmic phenomena.

Interestingly, Henshaw and his collaborators also find emissions from ionised gas coinciding with the arc cavity. This gas traced by radio emission exhibits a velocity consistent with the arc’s motion, which indicates a direct relationship between the hot ionised and the cold molecular gas.

“We explored several plausible scenarios that may account for an expanding envelope forming the arc,” Henshaw reports. He continues: “Comparing predictions from theory to our observations, we found that the stellar wind from a massive star of about 20 solar masses is likely the dominant mechanism.”

Altogether, the picture of an expanding bubble of hot gas driven by the stellar wind of a massive star formed inside the Brick is the most likely explanation for the arc‘s origin so far. This result puts the seemingly dormant Brick into a completely new perspective. Massive stars only rarely form in isolation. They usually mark the presence of an entire cluster of young stars of varying mass. If this were the case for the Brick, it might be less quiescent than previously thought.

To estimate the mass of the supposed group of stars, the astronomers simulated 10,000 clusters. A statistical analysis of those clusters with the most massive stars having 16 to 20 times the mass of the Sun points to a range of cluster masses between 400 and 700 solar masses. The authors of the underlying research article also demonstrated that with the currently available instrumentation, such clusters may easily be hidden in the confusion of the many stars detected towards the Galactic Centre and the obscuration of those stars by intervening gas and dust.

To better reveal the internal stellar population of the Brick, the astronomers are hoping for the James Webb Space Telescope still bound to launch in 2021. Those results will help them determine the underlying stellar population of the Brick and potentially locate the driving source of the arc.

Original publication

"A wind-blown bubble in the Central Molecular Zone cloud G0.253+0.016",

J. D. Henshaw, M. R. Krumholz, N. O. Butterfield, J. Mackey, A. Ginsburg, T. J. Haworth, F. Nogueras-Lara, A. T. Barnes, S. N. Longmore, J. Bally, J. M. D. Kruijssen, E. A. C. Mills, H. Beuther, D. L. Walker, C. Battersby, A. Bulatek, T. Henning, J. Ott, J. D. Soler, MNRAS, 2021, DOI: 10.1093/mnras/stab3039

Additional Information

The team consists of J. D. Henshaw (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), M. R. Krumholz (MPIA; Australian National University, Canberra, Australia; ARC Centre of Excellence for Astronomy in Three Dimensions, Canberra, Australia; ZAH, Universität Heidelberg, Germany), N. O. Butterfield (Villanova University, USA), J. Mackey (Dublin Institute for Advanced Studies, Ireland), A. Ginsburg (University of Florida, Gainesville, USA [UFL]), T. J. Haworth (Queen Mary University of London, UK), F. Nogueras-Lara (MPIA), A. T. Barnes (Universität Bonn, Germany), S. N. Longmore (Liverpool John Moores University, UK), J. Bally (University of Colorado, USA), J. M. D. Kruijssen (ARI, Universität Heidelberg, Germany), E. A. C. Mills (University of Kansas, Lawrence, USA), H. Beuther (MPIA), D. L. Walker (University of Connecticut, Storrs, USA [UCONN]), C. Battersby (UCONN), A. Bulatek (UFL), T. Henning (MPIA), J. Ott (National Radio Astronomy Observatory, Socorro, USA; New Mexico Institute of Mining and Technology, Socorro, USA), and J. D. Soler (MPIA; Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy).

The center of our Milky Way galaxy lies about 27,000 light-years away in the direction of the constellation of Sagittarius. At its core is a black hole about four million solar masses in size. Around the black hole is a donut-shaped structure about eight light-years across that rings the inner volume of neutral gas and thousands of individual stars. Around that, stretching out to about 700 light-years, is a dense zone of activity called the Central Molecular Zone (CMZ). It contains almost eighty percent of all the dense gas in the galaxy - a reservoir of tens of millions of solar masses of material - and hosts giant molecular clouds and massive star forming clusters of luminous stars, among other regions many of which are poorly understood. For example, the CMZ contains many dense molecular clouds that would normally be expected to produce new stars, but which are instead eerily desolate. It also contains gas moving at highly supersonic velocities - hundreds of kilometers per second (hundreds of thousands of miles per hours).

Where did the CMZ come from? No place else in the Milky Way is remotely like it (although there may be analogues in other galaxies). How does it retain its structure as its molecular gas moves, and how do those rapid motions determine its evolution? One difficulty facing astronomers is that there is so much obscuring dust between us and the CMZ that visible light is extinguished by factors of over a trillion. Infrared, radio, and some X-ray radiation can penetrate the veil, however, and they have allowed astronomers to develop the picture just outlined.

CfA astronomers Cara Battersby, Dan Walker, and Qizhou Zhang, with their team of colleagues, used the Australian Mopra radio telescope to study the three molecules HNCO, N2H+, and HNC in the CMZ. These particular molecules were selected because they do a good job of tracing the wide range of conditions thought to be present in the CMZ, from shocked gas to quiescent material, and also because they suffer only minimally from cluttering and extinction effects that complicate more abundant species like carbon monoxide. The scientists developed a new computer code to analyze efficiently the large amounts of data they had.

The astronomers find, consistent with previous results, that the CMZ is not centered on the black hole, but (for reasons that are not understood) is offset; they also confirm that the gas motions throughout are supersonic. They identify two large-scale flows across the region, and suggest they represent one coherent (or at most two independent) streams of material, perhaps even spiral-like arms. They also analyze the gas in several previously identified zones of the CMZ, finding that one shell-like region thought to be the result of supernova explosions may instead be several regions that are physically unrelated, and that a giant cloud thought to be independent is actually an extension of the large-scale flows. The scientists note that this work is a first step in unraveling an intrinsically complex galactic environment, and that pending research will trace the gas motions to larger distances and try to model the CMZ gas motions with computer simulations.

Original publication

"Molecular gas kinematics within the central 250 pc of the Milky Way,"

J. D. Henshaw, S. N. Longmore, J. M. D. Kruijssen, B. Davies, J. Bally,. Barnes, C. Battersby, M. Burton, M. R. Cunningham, J. E. Dale, A. Ginsburg, K. Immer, P. A. Jones, S. Kendrew, E. A. C. Mills, S. Molinari, T. J. T. Moore, J. Ott, T. Pillai, J. Rathborne, P. Schilke, A. Schmiedeke, L. Testi, D. Walker, A. Walsh and Q. Zhang, MNRAS, 2016, 457, 2675.