Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.
Title: Discovery of a Dense Association of Stars in the Vicinity of the Supermassive Black Hole Sgr A*
Authors: S. Elaheh Hosseini et al.
First Author’s Institution: University of Cologne and Max Planck Institute for Radio Astronomy
Status: Published in ApJ
The galactic center is a dynamic region at the heart of the Milky Way that is home to a supermassive black hole named Sgr A* (Sagittarius A-Star). It is densely packed with stars, gas, and dust, all interacting in fascinating ways. One challenge in studying this region is understanding the movement of stars, particularly those near the black hole, where gravitational forces are intense.
Astronomers have previously observed stars with unusual motions near Sgr A*, but a recent study highlights 42 sources near IRS 1W, an infrared source and well-known bow-shock source near the center of the galaxy. These sources are moving northward towards Sgr A*, and their behavior offers new insights into the region’s dynamics. What do the distributions, origins, and potential connections of these stars reveal about the enigmatic region surrounding Sgr A*?
What Are the N-Sources?
The study identifies these 42 sources, referred to as the N-sources (see Figure 1), as a group of stars moving northward relative to the central black hole. Their spatial distributions and proper motions reveal they are more clustered than stars in random regions at a similar distance from Sgr A*. This clustering, combined with their motion, suggests that the N-sources near the galaxy’s center may be part of a stellar association — a group of stars moving together in the same direction but not held together by gravity.
Figure 1: Image from 2005 showing the location of the N-sources and other IRS sources near the galactic center. The position of Sgr A* (the supermassive black hole) is indicated by the “x.” The N-sources are located about 6.05′′ from Sgr A* in this Ks-band image, which is sensitive to wavelengths of light slightly longer than visible light and is often used to observe regions obscured by dust. [Hosseini et al. 2024]
Figure 2: X-ray image from Chandra showing the position of Sgr A*, IRS 13, and other bright X-ray sources. The lack of a significant X-ray source at the position of the N-source association further supports the idea that these stars may not be part of a typical stellar cluster. [Hosseini et al. 2024]
Two Possible Origins
The study suggests two primary scenarios for the origin of the N-sources:
- A stellar cluster around an intermediate-mass black hole: One possibility is that the N-sources are part of a stellar cluster that is stabilized by an intermediate-mass black hole, a type of black hole with a mass between stellar-mass black holes and supermassive black holes like Sgr A*. The authors suggest the intermediate-mass black hole could have formed from stellar-mass black holes colliding with stars. If the N-sources are associated with this black hole, their motion could be influenced by its gravitational forces, keeping the cluster intact in the dense environment of the galactic center. Data from the Chandra X-ray Observatory suggest that this hypothetical intermediate-mass black hole would be surrounded by a faint accretion flow. Since weaker flows with less infalling material typically produce less X-ray radiation, this could explain the lack of bright X-ray emission at the N-source locations.
- A projection effect from a stellar disk: Alternatively, the N-sources and IRS 13, another stellar association, might simply appear clustered because of their position along the same stellar disk. This disk could be a projection of stars from a larger structure, such as the clockwise stellar disk observed in other regions near Sgr A*. In this case, the N-sources might not be physically bound to an intermediate-mass black hole but could just be stars from a local disk-like structure.
Looking Ahead: What’s Next for Studying the N-Sources?
To better understand the N-sources’ origin, future spectroscopic observations, especially with JWST, will be crucial. These observations could provide key data on the stars’ velocities, helping astronomers determine whether they are part of a stellar cluster around an intermediate-mass black hole or just a projection from a larger stellar disk.
The study of the N-sources offers important insights into the dynamics of the galactic center, a region that is both an astrophysical laboratory and a challenging environment for astronomers. By understanding how stars move in this dense, gravitationally extreme region, researchers can learn more about the role of supermassive black holes, intermediate-mass black holes, and stellar clusters in shaping the structure and evolution of our galaxy.
Original astrobite edited by Viviana Cáceres and Lucas Brown.