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High-energy x-ray sources (blue) are seen near the core of the Milky Way (bright region at lower right) in this composite image.


A swarm of black holes may be lurking in our galaxy’s heart

Hundreds of black holes may lie at the center of our Milky Way galaxy, according to a new study. Such a tight swirl of black holes, which had been theorized for decades but never detected, bolsters current models of how galaxies evolve, scientists say.

Many galaxies, including our own, have one supermassive black hole at their core, which grows by slowly pulling in a host of smaller objects, including stars and entire star systems. Scientists have suspected that this core region may also contain numerous smaller black holes tightly orbiting the supermassive one, but they’ve lacked evidence of such a swarm—until now.

In the new study, Charles Hailey, an astrophysicist at Columbia University, and his colleagues scrutinized the past dozen years of data gathered by the Chandra X-ray Observatory, an orbiting craft whose instruments are designed to detect high-energy radiation emitted by the immensely hot material surrounding exploded stars and near black holes. When they looked at the region of space within about 12 light-years of our galaxy’s supermassive black hole, an object dubbed Sagittarius A*, they found hundreds of x-ray sources. And when they compared the x-ray emissions for those closest to Sagittarius A* with those a little farther away, they found big differences.

For example, Hailey notes, several x-ray sources within 3.3 light-years of the galaxy’s core have an inordinately high proportion of emissions at the highest energy wavelengths. Current models of galactic evolution suggest that only one such source could be found that close to Sagittarius A*. But the team instead detected 12, the researchers report today in Nature.

Artist’s representation of an X-ray–emitting binary system that includes a black hole.

NASA/Wikimedia Commons

At least six of those x-ray sources—and possibly all 12—are likely to be what astronomers call x-ray binaries, Hailey says. Typically, one member of the pair is a garden-variety star while the other is either a black hole or a neutron star. However, emissions from x-ray binaries that include neutron stars often surge suddenly and then subside at least once every 5 to 10 years, Hailey explains. Because the x-ray emissions from their sources haven’t varied in the past 12 years, Hailey presumes that these binaries include small-mass black holes.

“This is a small number of sources, but they’re very intriguing,” says Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena who was not involved with the work. The balance of high-energy versus low-energy x-rays emitted by these sources “are consistent with those from low-mass binaries with black hole companions,” she notes.

In our neck of the galaxy, x-ray binary systems aren’t so common. But for every one such system astronomers have spotted, they’ve also detected many more black holes that don’t have companions. Such isolated black holes would be too dim to discern at the galactic core, but the x-ray binaries serve as a tracer suggesting they’re there—and in really big numbers. Even if only six of the x-ray sources include a black hole, there are probably between 300 and 500 solo black holes orbiting within 3.3 light-years of the galactic core, Hailey and his colleagues figure.

The work may also help shed light on how x-ray binaries form and develop, Harrison says. For instance, in the crowded heart of a galaxy, black holes may have more opportunities to pair up with nearby stars—and then slurp material from them, generating x rays in the process—than they do in sparser regions of the star group. “There’s a lot of uncertainty about how these things form,” she notes.