Milky Way: Tens of Thousands of Black Holes Exist at Our Galaxy's Center, Study Suggests
Scientists have long predicted that surrounding each of the supermassive black holes (SMBH) at the center of every large galaxy, including our own Milky Way, are thousands of smaller black holes.
However, after two decades of extensive searching, researchers have found little credible evidence to support this theory. But a new study, published in the journal Nature, may buck the trend, providing the first credible evidence that the prediction was correct.
In the study, a team of astrophysicists led by researchers from Columbia University describe how they discovered 12 black holes within three light-years of Sagittarius A* (Sgr A*)—the SMBH at the center of the Milky Way—using data from NASA’s Chandra X-ray Observatory.
Extrapolating from this discovery, they then estimated that, in total, there must be around 10,000 isolated black holes and between 300 and 500 binary black hole systems—two-object star systems in which one of the components is a black hole—within three light-years of the Galactic Center.
“These results confirm a decades-old prediction that large numbers of black holes should be swarming in orbit around the supermassive black hole,” Columbia astrophysicist Chuck Hailey told Newsweek. “It was a pretty tantalizing mystery: Why are we missing this huge population of black holes?”
According to Hailey, the Milky Way is the only place we can really test the prediction because smaller black holes surrounding SMBHs in other galaxies are too far away and too close to their galactic centers to effectively study with current technology.
But even in the Milky Way, finding black holes around the Galactic Center is tricky, despite the predictions, because they are so hard to detect.
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“Black holes are black for a reason,” Hailey said. “They don’t do much that lends to easy detection. They eat gas and dust but are not good at turning this black hole food into X-rays, or any other signal we can detect with telescopes.”
So to determine the number around the Galactic Center, the researchers had to delve into their knowledge of black hole physics.
Hailey explained that Sgr A* is surrounded by a vast halo of dust and gas, which is a prime location for massive stars to form. We know that when these massive stars die, they turn into black holes, which is what led to the theory that there should be thousands of these objects surrounding the Galactic Center.
Furthermore, scientists think that black holes from beyond the halo can gradually be pulled closer to the SMBH as they lose energy, adding to the number. Most of the black holes remain isolated, but a small minority become attached to the orbits of low mass stars in the region, forming a binary system. This phenomenon is helpful to the Columbia researchers because it makes the black holes detectable with observatories like Chandra.
“It is easier to see [the black holes] if they have a companion star, since gas from the companion makes a disk around the black holes, which is good at emitting X-rays,” Hailey explained. “Sometimes these binaries emit big bursts of X-rays, but these are very rare, so we have not had luck with this approach.”
“So the best bet is to look at the steady X-ray emission from the black hole binaries, but these are less numerous. And the Galactic Center is so crowded with X-ray-emitting objects and X-ray glowing gas that it is hard to find the black hole binaries that emit X-rays,” he continued. “So you have to separate out the boring X-ray sources from the fun stuff—the black holes. It's not easy to distinguish one type of source from another.”
Despite the difficulties, the team was able to identify 12 black holes. And because scientists know what fraction of black holes will bind with low mass stars, after they discovered the 12 they were able to calculate the population of isolated black holes around Sgr A*.
Hailey said the new findings could significantly advance research into one of the most exciting fields of astronomy right now: gravitational waves.
“The observations give hard evidence for the number and distribution of black holes in space, so theorists can use this information to revise their estimates of how many gravitational wave events associated with black holes may happen in the centers of galaxies,” he said. “That’s nice, because all eyes are on the gravitational wave observatories now.”