Milky Way Core May Be Riddled With 10,000+ Black Holes

Columbia astrophysicists have discovered 12 black hole-low mass binaries orbiting Sgr A* at the center of the Milky Way galaxy. Their existence suggests there are likely about 10,000 black holes within just three light years of the Galactic Center. Columbia University

We’ve only ever known about one black hole in the center of the Milky Way. As wide as the solar system and packing a mass of 4 million suns, it’s called Sagittarius A* (pronounced ‘A star’).  Now, it looks like it’s got a lot of company. A Columbia University-led team of astrophysicists has discovered a dozen smaller black holes near this proverbial Jabba the Hut. Extrapolating from the Chandra X-ray Observatory observations, there could easily be 10,000 black holes within three light years of the core. Astronomers are jazzed by the finding because it finally provides an opportunity to see how big black holes interact with little black holes.

“The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones because we simply can’t see their interactions in other galaxies,” said Columbia Astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab and lead author on the study. “In a sense, this is the only laboratory we have to study this phenomenon.” The results appeared in an April 5 paper in the Nature.

This composite photos shows the center of the Milky Way in X-rays (blue) photographed the Chandra Observatory and in infrared by the Hubble Space Telescope. The inset shows a close-up view of Sagittarius A* (Sgr A*) only in X-rays, covering a region half a light year wide. The diffuse glow is from hot gas captured by the black hole and being pulled inwards. NASA/Chandra

For more than two decades, researchers have searched unsuccessfully for evidence to support a theory that thousands of black holes surround supermassive black holes (SMBHs) at the center of large galaxies. I was amazed to learn that we know of only about 60 black holes in the entire galaxy, yet there are supposed to be 10,000 to 20,000 of them in the six-light-year region centered on Sgr A*. Despite previous efforts to find them, there’s been little evidence of their existence until now.

Sgr A* is surrounded by a halo of gas and dust that provides the necessary material for the birth of massive stars, which often end their lives in supernova explosions that produce black holes. Additionally, black holes from beyond the core are gravitationally nudged by Sgr A*, causing them to lose their energy and draw closer to the galactic center, where they’re held captive by the supermassive hole’s gravitational might. For both these reasons, astronomers expected black holes to be common within the inner few light years of the core. It turns out they are, but scientists had to look for them in a new way.

In this artist’s view, multi-million degree gas swirls down Sgr A*, a black hole with about 4 million times the mass of the Sun. At just 26,000 light years from Earth, Sgr A* is one of very few black holes  where we can actually witness the flow of matter nearby. Now, scientists can study how nearby small black holes interact with the larger one. NASA/Chandra

While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary. Researchers believe there is a heavy concentration of these isolated and “mated” black holes in the galactic center. They hoped to spot them when the close binary shot out a bright burst of X-rays, but these events only occur once every 100 to 1,000 years. Instead, the team decided to look for the difficult but sure thing: the fainter but steadier X-rays the pairs give off when they’re quietly going about their business.


“If we could find black holes that are coupled with low mass stars and we know what fraction of black holes will mate with low mass stars, we could scientifically infer the population of isolated black holes out there,” said Hailey.

Hailey and colleagues searched for X-rays dribbled out by black hole-low mass binaries in their inactive state and were able to find 12 within three light years, of Sgr A*. The researchers then extrapolated from their observations that there must be anywhere from 300 to 500 black hole-low mass binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.

Think of all vacuuming that must be happening within the inner few light years of the Milky Way’s center. Anything straying too close to any of these black holes would be taken down the rabbit hole never to be seen again. What a bizarre and alien environment!

(Some material for this blog was taken and re-worked from the Columbia News.)