Multiple Black Holes may circle Sagittarius A (Sgr A*)

We now learn that multiple black holes may circle the super massive black hole at the center of our Milky Way Galaxy, known as Sagittarius A (Sgt A*).

Columbia Astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab says: "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."

"There are only about five dozen known black holes in the entire Milky Way Galaxy -- 100,000 light years wide -- and there are supposed to be (according to theory) 10,000 to 20,000 of these things in a region just six light years wide that no one has been able to find," Hailey said, adding that extensive fruitless searches have been made for black holes around Sgr A*, the closest SMBH {Super Massive Black Hole: a black hole containing a mass of the order of hundreds of thousands to billions of times the mass of the Sun (M☉)} to Earth and therefore the easiest to study. "There hasn't been much credible evidence."

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 gravitational force.

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 orbiting the Galactic Center, forming a density cusp, or ring, which gets more crowded as distance to the SMBH decreases.

"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," explained Hailey.

Hailey and his 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 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 binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.

This is an incredible identification, especially one based stored data from a past satellite's findings.

"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."

Hailey's co-authors on the paper include: Kaya Mori, Michael E. Berkowitz, and Benjamin J. Hord, all of Columbia University; Franz E. Bauer, of the Instituto de Astrofísica, Facultad de Física, Pontificia, Universidad Católica de Chile, Millennium Institute of Astrophysics, Vicuña Mackenna, and the Space Science Institute; and Jaesub Hong, of Harvard-Smithsonian Center for Astrophysics.milky way over pyramids.jpg

When you visualize the center of our galaxy, the Milky Way, what the Romans called the Via Lactea, which stretches across our night sky in its beautiful multi-hued band, we now know that's its center is a place of roiling destruction, of deadly X-rays, of black holes devouring the stars they orbit as they, too, are in a ceaseless death orbit around a super massive black hole known as Sagittarius A.