Last updated April 5, 2018 at 11:00 pm
Astronomers have announced the discovery of small black holes near the centre of the Milky Way, and think there could be 10,000 more.
For a long time we’ve known about the supermassive black hole at the very centre of our galaxy. However, US scientists have just announced the discovery of an entourage of smaller black holes joining it in the heart of the Milky Way.
Astronomers and astrophysicists have long predicted that the supermassive black hole Sagittarius A* (Sgr A*) was surrounded by clusters of these stellar-mass black holes.
While we’ve known there have been a bevy of other objects trapped in its outer reaches – neutron stars, white dwarfs and the such – the detection of these small black holes has been elusive, until now.
Using over a decade of data from the Chandra X-ray Observatory, Charles Hailey from Columbia University and a team of researchers found a dozen inactive low-mass binary systems where a black hole and another object, such as a star, orbit each other.
These 12 black holes were all within a parsec (approximately 3.3 light years) of Sgr A*. And that distribution could mean there are thousands more in the area, waiting to be found.
Thousands of black holes along for the ride
Sgr A* is surrounded by a region 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 further away could fall under the influence of the supermassive black hole as they lose their energy and drift inwards. Over time, they slowly get pulled into the vicinity of the supermassive black hole, where they then are held captive by its force.
Most of these trapped black holes will remain isolated, orbiting a lonely path. However, some will capture and bind to a passing star, forming their own stellar binary.
Based on the properties and spatial distribution of the 12 binary systems, the researchers think there could be anywhere from 300 to 500 black hole-low mass binaries, and about 10,000 isolated black holes in the close area surrounding Sgr A*.
This collection forms a “density cusp”, with a heavy concentration of these isolated and mated black holes which get more crowded as distance to the galactic centre decreases.
“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 centre 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 centre of the galaxy.”
Finding a black hole
One of the difficulties of studying black holes is finding them in the first place, and that’s where previous searches for these companion black holes have failed.
Other studies have focused on looking for the bright burst of X-ray glow that sometimes occurs in black hole binary systems.
“It’s an obvious way to want to look for black holes,” Hailey said, “but the Galactic Centre 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.”
Despite numerous telescopes trained on Sgr A*, this rarity of outbursts meant the elusive black holes remained unseen.
Taking a new approach, the team looked for the fainter, but steadier, X-rays emitted when the binaries 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.”
To test whether this technique would yield any success, the researchers used archival data from the orbiting Chandra X-ray Observatory. Chandra has become invaluable for studies such as this, as most galactic X-rays are absorbed by the Earth’s atmosphere making them undetectable to ground-based telescopes.
The next difficulty was filtering out the signals from non-black hole sources. The team searched instead for lower-energy X-rays, which are a signature of emissions from binary systems with a black hole.
Using this technique, they found the twelve, previously unseen, black hole binaries around the galactic centre. And from that, they estimate between 10,000 and 20,000 others nearby.
The need to keep searching
One downside to this technique however, is the exceedingly dim signals being detected by the Chandra Observatory.
Over such a long distance it has been estimated only one photon from the binary systems would be detected by Chandra every 5 to 10 minutes.
And that has some astronomers calling for caution, including Hailey himself.
Mark Morris from the University of California, who was one of the first to propose the presence of these thousands of black holes in 1993, told Scientific American that there was a chance some of the photon detections could be an anomaly caused by coincidental emissions from other sources.
Hailey himself agrees, saying he only felt certain half of the signals were from black holes. There is a chance the other six could be from sources such as rapidly spinning neutron stars.
Nevertheless, astronomers are predicting that this discovery will have profound effects in the future. Follow up studies will be carried out in the future using more data from Chandra and any potential successors, but this paper could be the start of an exciting time in galactic research by opening up a myriad of opportunities to better understand the universe.
The research has been published in Nature