New research shows that a past collision between the Milky Way’s central black hole and a smaller black hole could explain the dynamics of the S-stars, a group of stars that orbit precariously close to our galaxy’s supermassive black hole.

Journey to the Center of the Milky Way

The center of the Milky Way harbors a supermassive black hole (Sagittarius A* or Sgr A*) with a mass of 4 million Suns. The nearest neighbors of this behemoth are a compact disk of massive young stars and a collection of stars called the S-stars, which inhabit the innermost 48 light-days of our galaxy.

While the stars in the disk are orderly, arranged on orbits with moderate eccentricities and low inclinations, the S-stars orbit Sgr A* every which way, careening around the black hole with a wide range of eccentricities and inclinations. So far, the cause of this unusual distribution of stars is unknown.

When Black Holes Collide

Past research has struggled to explain the S-stars’ orbits. A successful theory of S-star origins must account for the stars’ eccentric orbits (e = 0.61, on average) and high orbital inclinations (i = 79º, on average), and it must produce these characteristics within the 15-million-year lifetime of the stars.

In a recent research article, a team led by Tatsuya Akiba (University of Colorado Boulder) offered a new hypothesis to explain these traits: Sgr A* absorbed a smaller black hole in the not-so-distant past, and the aftermath of the merger created the distribution of stars seen today.

The premise is not far-fetched: at the ripe old age of nearly 14 billion years, the Milky Way has likely gulped down neighboring dwarf galaxies and globular clusters multiple times. When a black hole embedded within one of these meals of stars and gas merges with the Milky Way’s central black hole, the collision causes Sgr A* to recoil — potentially rearranging the stars in its vicinity in the process.

Radical Reorganization

Akiba and collaborators used N-body simulations to explore the outcomes of such a collision. In their simulations, Sgr A* is situated within an axisymmetric disk of stars or gas. A smaller black hole falls toward Sgr A*, spirals inward, and merges with the larger black hole. The asymmetric emission of gravitational waves causes Sgr A* to recoil, warping the surrounding collection of stars and gas into an eccentric disk. For the eccentricity of the disk to match what is seen in the disk of stars surrounding Sgr A* today, the incoming black hole must have a mass of roughly 200,000 solar masses.

After the merger, the stars orbiting farther out torqued the inclinations and eccentricities of the innermost stars up to high values through what’s called the eccentric Kozai–Lidov mechanism. After 2 million years of simulation time, this interaction produced a stellar distribution similar to what is seen today: a disk of moderate eccentricity surrounding “S-stars” with eccentric and highly inclined orbits.

While the properties of the modeled S-stars don’t exactly match the properties of the real deal — the simulated stars have, on average, lower inclinations and eccentricities — the authors noted that this work represents a first foray into their hypothesis. Modeling that explores a broader range of parameter space is needed to fully understand the origins of the stars at the center of the Milky Way.

Citation

“On the Formation of S Stars from a Recent Massive Black Hole Merger in the Galactic Center,” Tatsuya Akiba et al 2025 ApJL 987 L27. doi:10.3847/2041-8213/addc5d

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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.

The authors also analyzed X-ray data from the Chandra X-ray Observatory, revealing no significant X-ray emission from the N-source region (Figure 2). In contrast, both the X-ray bright Sgr A* and IRS 13, another well-known infrared source near this region, show significant emission. This lack of X-ray emission from the N-sources adds complexity to understanding these sources and their origin.

Two Possible Origins

The study suggests two primary scenarios for the origin of the N-sources:

1. 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.
2. 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.

Original astrobite authored by Sparrow Roch.

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