Tim Hornyak

Sep 18th 2020

How Many Black Holes Are in the Milky Way?


In his book “A Brief History of Time,” the late cosmologist Stephen Hawking included a now-famous bit of speculation about how a fictional astronaut would get decidedly longer and thinner when crossing a black hole’s event horizon, from which nothing escapes. Steering clear of the black hole at the center of the Milky Way is one thing, but scientists now suggest it’s surrounded by some 10,000 other black holes like a giant piece of Swiss cheese. Much more than simply helping us answer the question, “How many black holes are in the Milky Way?,” the finding could be the first in a set of space discoveries centered on black hole swarms.

Black Hole Signatures

In a study published in Nature, a team led by Columbia University researchers reported discovering a dozen black holes surrounding the supermassive black hole at the center of the Milky Way. They were working on the premise of a decades-old theory that thousands of black holes — both isolated and ones paired with stars — should be grouped in a “density cusp” around supermassive black holes in the heart of big galaxies. noted that these black holes have masses that are millions or billions times that of our sun.

Many unsuccessful attempts to find these black holes sought signs of the bright, fleeting X-ray bursts that occur when black holes pair up with low-mass stars. The researchers, led by Columbia Astrophysics Lab co-director Chuck Hailey, used a different technique. They instead focused on searching for fainter but more consistent X-rays emitted after the stellar pairings, according to Science Daily.

Using archival data from a NASA telescope known as the Chandra X-ray Observatory, Hailey and colleagues looked for these fainter X-ray signatures and found 12 within three light years of the Milky Way’s supermassive black hole, which is known as Sagittarius A*. Isolated black holes don’t give off these telltale signs, but knowing what proportion of black holes pair with stars allowed the team to extrapolate how many there are in the region. They estimate 300 to 500 black holes paired with stars and roughly 10,000 isolated black holes around Sagittarius A*, said Science Daily.

Impact on Gravitational Waves

The discovery could have a significant impact on our understanding of the origins and dynamics of the galaxy, just as the detection of the most distant black hole ever recorded is shedding new light on the early stages of the cosmos. For instance, a multiplicity of black holes increases the chances of a collision, an event that can generate gravitational waves like the kind detected in 2016 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which confirmed a century-old prediction by Albert Einstein and helped us better answer the question of how many black holes are in the Milky Way.

“This finding confirms a major theory and the implications are many,” Hailey told Columbia News. “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.”

Since the Milky Way’s density cusp is relatively close to Earth compared to the centers of distant galaxies, the research may also deepen our understanding of how large black holes interact with smaller ones.

“‘A discovery like this will always have consequences that we cannot presently predict,'” Jordi Miralda-Escudé, an astrophysicist at the University of Barcelona, told Scientific American. “‘If confirmed, the existence of these black holes suggests similar concentrations should exist in the centers of most galaxies throughout the universe.'”