Key takeaways

  • Black Holes at the Galaxy’s Core: Scientists have found evidence of hundreds of black holes near the Milky Way’s supermassive black hole, Sagittarius A*.
  • Confirmation of Predictions: This discovery supports long-standing theories that a swarm of smaller black holes surrounds the central supermassive black hole in galaxies.
  • X-ray Evidence: Using data from the Chandra X-ray Observatory, researchers detected high-energy x-ray sources within 12 light-years of Sagittarius A*.
  • X-ray Binaries: Many of these x-ray sources are likely x-ray binaries, systems where a regular star orbits a black hole or neutron star, providing clues to the presence of hidden black holes.
  • Galactic Evolution Insight: The findings enhance our understanding of galactic evolution, showing how black holes and stars interact and evolve in the dense core of a galaxy.

A recent study suggests that hundreds of black holes may exist at the heart of our Milky Way galaxy. This tight swirl of black holes, which had been predicted for decades but never observed, strengthens current models of how galaxies grow, according to astronomers.

Many galaxies, including our own, have a single supermassive black hole at their core, which expands by slowly attracting a variety of smaller objects, including stars and whole star systems. Scientists have believed that this core area contains a swarm of lesser black holes surrounding the supermassive one, but there has been no confirmation of such a swarm until now.

In the new study, Charles Hailey, an astrophysicist at Columbia University, and his colleagues examined data collected by the Chandra X-ray Observatory, an orbiting craft whose instruments are designed to detect high-energy radiation emitted by the extremely hot material surrounding exploded stars and black holes. When they looked at the region of space within around 12 light-years of our galaxy’s supermassive black hole, known as Sagittarius A*, they discovered hundreds of x-ray sources. When scientists compared the x-ray emissions of individuals closest to Sagittarius A* with those farther away, they discovered significant discrepancies.

For example, Hailey observes that some x-ray sources within 3.3 light-years of the galaxy’s center have an unusually high fraction of emissions at the highest energy wavelengths. According to current galactic development models, only one such source may exist around Sagittarius A*. However, the team found 12, according to a paper published in Nature.

According to Hailey, at least six of the x-ray sources, if not all twelve, are likely to be x-ray binaries. Typically, one member of the pair is a regular star, while the other is either a black hole or a neutron star. However, Hailey argues that emissions from x-ray binaries containing neutron stars frequently peak and then diminish every 5 to 10 years. Hailey believes that these binaries contain small-mass black holes since their x-ray emissions have remained constant over the last 12 years.

“This is a small number of sources, but they’re very intriguing,” says Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena who was not involved with the work. The balance of high-energy versus low-energy x-rays emitted by these sources “are consistent with those from low-mass binaries with black hole companions,” she notes.

In our galaxy, x-ray binary systems are uncommon. However, for every such system discovered by astronomers, there are many more black holes without partners. Such single black holes would be too faint to see at the galactic center, but the x-ray binaries operate as a tracer, indicating their presence—and in large numbers. Even if just six x-ray sources include a black hole, Hailey and his colleagues estimate that there are 300 to 500 solo black holes circling within 3.3 light-years of the galactic core.

Harrison believes the research might potentially provide light on how x-ray binaries emerge and develop. For example, at the dense core of a galaxy, black holes may have more opportunity to partner up with neighboring stars—and then suck material from them, generating x rays in the process—than in sparser portions of the stellar group. “There’s a lot of uncertainty about how these things form,” she says.

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