Study: Tens of Thousands of Black Holes May Be at Milky Way’s Center

The Milky Way. Photo credit: ESO/Y. Beletsky

(CN) – A new study supports the prevailing theory of where some of the universe’s most mysterious inhabitants – black holes – reside.

For more than two decades, scientists have searched unsuccessfully for evidence that thousands of black holes surround supermassive black holes at the centers of large galaxies.

A Columbia University-led team of astrophysicists is the first to corroborate the theory.

Their findings, published Wednesday in the journal Nature, open up myriad avenues to better understanding the universe.

Succeeding where others have failed, the scientists discovered a dozen black holes congregated around Sagittarius A*, the supermassive black hole in the heart of the Milky Way Galaxy.

“Everything you’d ever want to learn about the way big black holes interact with little black holes, you can learn by studying this distribution,” said lead author Chuck Hailey, co-director of the Columbia Astrophysics Lab. “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.

“In a sense, this is the only laboratory we have to study this phenomenon.”

Hailey noted that despite the theory that thousands of black holes should be close to each other, relatively few of the celestial bodies have actually been discovered in the Milky Way.

“There are only about five dozen known black holes in the entire galaxy – 100,000 light-years wide – and there are supposed to be 10,000 to 20,000 of these things in a region just six light-years wide that no one has been able to find,” Hailey said. “There hasn’t been much credible evidence.”

Sagittarius A* is surrounded by a halo of dust and gas that offers the ideal breeding ground for the birth of massive stars, which live, die and could transition into black holes there, Hailey said. Furthermore, black holes from outside the halo are believed to fall under the influence of a supermassive black hole as their energy diminishes, prompting them to be pulled near the celestial giant and held by its force.

While most trapped black holes remain isolated, some catch and bind to a passing star, forming a stellar binary. Scientists believe there is a strong concentration of these mated and isolated black holes in the galactic center, the rotational center of the Milky Way, forming a density cusp – a localized increase in number – which gets more congested as distances to the supermassive black hole decreases.

In the past, failed efforts to identify evidence of such a cusp have concentrated on searching for the bright burst of X-ray glow that periodically occurs in black hole binaries.

“It’s an obvious way to want to look for black holes,” Hailey said, “but the galactic center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years.”

With this in mind, the team realized that to detect black hole binaries, they would have to look for the fainter but steadier X-rays emitted when the celestial pairs are in an inactive state.

“It would be so easy if black hole binaries routinely gave off big bursts like neutron star binaries do, but they don’t, so we had to come up with another way to look for them,” Hailey said. “Isolated, unmated black holes are just black – they don’t do anything. So looking for isolated black holes is not a smart way to find them either. But when black holes mate with a low-mass star, the marriage emits X-ray bursts that are weaker, but consistent and detectable.

“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.”

The team turned to archival data from the Chandra X-ray Observatory to test their approach. They looked for X-ray signatures of black hole-low mass binaries in their inactive state and managed to find 12 within three light-years of Sagittarius A*.

They then analyzed the spatial distribution and properties of the identified binary systems and concluded from their observations that there must be anywhere from 300 to 500 such binaries and roughly 10,000 isolated black holes in the area surrounding Sagittarius A*.

“This finding confirms a major theory and the implications are many,” said Hailey. “It is going to significantly advance gravitational wave research because knowing the number of black holes in the center of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them.

“All the information astrophysicists need is at the center of the galaxy.”

The research was funded by NASA.


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