Tens of Thousands of Black Holes Found in the Milky Way

Milky Way
Image Credit: Graham Holtshausen

The image above shows the Milky Way stretching across the sky, with Sgr A*, the area of space that contains both the center of the galaxy and a super massive black hole, directly above the tallest peak.

While it has long been known that the area around the core of the Milky Way contains a super massive black hole known as Sgr A*, it has also long been suspected that the same area must also contain hundreds, if not thousands of smaller black holes. However, since these smaller astronomical objects are for all practical purposes invisible until they interact with something else, proving their existence has been a sort of Holy Grail for many astronomers.

Nonetheless, a team of Columbia University researchers under the leadership of astrophysicist Chuck Hailey announced in a study published on April 5th in an issue of the journal Nature that they have found the solution to their quest, and that the center of the Milky Way does indeed contain many thousands of small, low-mass black holes.

According to Hailey, only about 60 or so black holes are known to exist our entire Milky Way that spans more than 100,000 light years, but also according to the scientist accepted models predict that the galaxy’s innermost six light years surrounding Sgr A* should contain thousands of smaller black holes. In the past, however, conventional methods of finding the elusive black holes have failed to turn up any, which forced the team of researchers to approach the problem in a novel way.

At the heart of the solution is the fact that since Sgr A* is surrounded by a dense halo of gas and dust, massive stars should inevitably be born here, and since many of these stars are massive enough to morph into black holes when they die, Sgr A* should hold many, if not most of these black holes captive. Moreover, many black holes from outside of the halo would also have been captured by Sgr A; therefore, most astronomers believe that the density of the collection of black holes should increase with decreasing distance to Sgr A*.

Since most of these black holes would be isolated, though, they would remain invisible in X-rays because they don’t interact with anything. By way of contrast, massive black hoes become visible when that they emit short-lived, but powerful X-ray bursts when they interact with other objects. Thus, using this as a point of departure, the research team started looking for black holes that have become attached to passing stars, since the mating would produce steady, albeit relatively low-energy X-ray emissions for as long as the marriage lasted.

By examining archival data from the Chandra X-ray Observatory to locate these binary systems, the research team found 12 such stellar/black hole binary systems within three light years of Sgr A* almost immediately. Based on a complex analysis of the properties and spatial distribution of the confirmed binary systems, the team then calculated that the area surrounding Sgr A* must contain at least 300 to 500 similarly-attached black holes, in addition to about 10,000 or so black holes of various masses that exist in isolation.

Summing up the study, Hailey stated, “This finding confirms a major theory and the implications are many. 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 center of the galaxy.”

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