“There are only about five dozen known black holes in the entire galaxy—120,000 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,” said Columbia University astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab, adding that extensive fruitless searches have been made for black holes around Sgr A*, the closest SMBH to Earth and therefore the easiest to study. “There hasn’t been much credible evidence.”
A decades-old prediction
In 2018, a team of astrophysicists has discovered a dozen black holes gathered around Sagittarius A* (Sgr A*), the supermassive black hole in the center of our Milky Way Galaxy. The finding was the first to support a decades-old prediction, opening up myriad opportunities to better understand the universe.
“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 Hailey, lead author on the study. “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.”
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.
Perfect breeding ground for the birth of massive stars
Hailey explained that Sgr A* is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars, which live, die and could turn into black holes there. Additionally, black holes from outside the halo are believed to fall under the influence of the SMBH as they lose their energy, causing them to be pulled into the vicinity of the SMBH, where they are held captive by its force.
A Density Cusp Forms Closer to Sgr*A
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, forming a density cusp which gets more crowded as distance to the SMBH decreases. In the past, failed attempts to find evidence of such a cusp have focused on looking for the bright burst of X-ray glow that sometimes 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.” To detect black hole binaries then, Hailey and his colleagues realized they would need to look for the fainter, but steadier X-rays emitted when the binaries are in an inactive state.
In explaining the concentration or migration of balck holes around the Milky way’s supermassive black hole, Harvard astrophysicist, cwrote in an email to The Daily Galaxy, “the dynamical interaction (or friction) of stars around the central massive black hole leads to central migration of relatively heavy stellar remnants, meaning black holes (BHs). At a risk of gross simplification with an inaccurate analogy, I might say it is basically like heavy objects sinking deeper than lighter objects.”
“Now we know that most galaxies, if not all,” continues Hong, “have a supermassive black hole, it is quite plausible that the BH concentration is widely occurring throughout the Universe.”
Inferring the population of isolated black holes
“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,” says Hailey. “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.”
Revealed by Chandra X-ray Observatory Data
Hailey and colleagues turned to archival data from the Chandra X-ray Observatory to test their technique. They searched for X-ray signatures of black hole-low mass star binaries in their inactive state and were able to find 12 within three light years, of Sgr A*. The researchers then analyzed the properties and spatial distribution of the identified binary systems and extrapolated from their observations that there must be anywhere from 300 to 500 black hole-low mass star binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.
“While many theories have been proposed, one likely explanation is that the Galactic Center is subject to episodic periods of intense star formation on a timescale of ten million years or so,” Hailey wrote in an email to The Daily Galaxy. “These can produce the massive stars which ultimately form stellar mass black holes in the vicinity of the supermassive black hole Sgr A*. It is likely that similar processes produce black hole cusps around supermassive black holes in other galaxies, although finding evidence for such cusps at such distances is challenging.”
“This finding confirms a major theory and the implications are many,” Hailey said. “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.”
Source: Charles J. Hailey et al, A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy, Nature (2018). DOI: 10.1038/nature2502