In the heart of our home, the Milky Way galaxy, a black hole with a mass of 4.3 millions suns grows. Now, it has company. A lot of company. Astronomers have discovered evidence for thousands of “bite-sized” black holes located near the center of our Milky Way galaxy using data from NASA’s Chandra X-ray Observatory.
These are stellar-mass black holes, which typically have between five to 30 times the mass of the sun, and they’re found within three light years of Sagittarius A* (Sgr A*), the supermassive black hole at the galaxy’s center. From studies of stars in other galaxies, astronomers determined that smaller black holes could slowly drift toward the core and settle there. It’s estimated that as many 20,000 black hole may be gathered around Sgr A*.
Black holes, the remnants of supernova explosions and star mergers, are completely invisible. A black hole gives itself away when it pairs up closely with another star and pulls gas from it. The gas falls into a disk around the hole and heats up through friction to millions of degrees. Hot gas radiates X-rays before it disappears down the maw. Astronomers call these X-ray binaries.
A team of researchers, led by Chuck Hailey of Columbia University in New York, used Chandra data to search for X-ray binaries and detected a dozen X-ray binaries within three light years of the core that likely contain black holes. Since only the brightest X-ray binaries containing black holes are likely to be detectable at Sgr A*’s distance of about 25,000 light years, their study implies there are many, many more. At least 300 and up to a thousand.
Black holes with companion stars near Sgr A* could provide insight into how X-ray binaries form from close encounters between stars and black holes. They may also inform future gravitational wave research. Gravitational waves, literally ripples in the fabric of spacetime, are created when massive stars or black holes merge. 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.
Keep in mind, we’re only seeing the black holes that have companion stars. There should be lots more without companions that would be — for now — undetectable. According to follow-up work by Aleksey Generozov of Columbia and his colleagues, more than about 10,000 black holes and as many as 40,000 black holes should exist in the center of the galaxy. While black holes seem to best explain the dozen sources found in the sky, the researchers caution that about half of them could be rapidly spinning neutron stars — tiny, incredibly dense, city-sized stars that are the remnant cores of supergiant stars that explode as supernovas.
Here’s their research paper that appeared recently in the journal Nature.