Astronomers using NASA’s Chandra X-ray Observatory have discovered what they believe are 12 stellar-mass black holes gathered around Sagittarius A*, the supermassive black hole in the Milky Way’s center, leading them to conclude there are probably 10,000 such objects spread throughout the central region of our Galaxy. Their work appears in the April 5, 2018 issue of the journal Nature.
The region around the center of our Milky Way Galaxy glows colorfully in this image taken by NASA’s Spitzer Space Telescope. In the image, the myriad of stars crowding the center of our Galaxy creates the blue haze that brightens towards the center of the image. The green features are from carbon-rich dust molecules, called polycyclic aromatic hydrocarbons, which are illuminated by the surrounding starlight as they swirl around the Galaxy’s core. The yellow-red patches are the thermal glow from warm dust. The polycyclic aromatic hydrocarbons and dust are associated with bustling hubs of young stars. These materials, mixed with gas, are required for making new stars. The brightest white feature at the center of the image is the central star cluster in our Galaxy. At a distance of 26,000 light years away from Earth, it is so distant that, to Spitzer’s view, most of the light from the thousands of individual stars is blurred into a single glowing blotch. The region pictured here is immense, with a horizontal span of 2,400 light-years (5.3 degrees) and a vertical span of 1,360 light-years (3 degrees). Though most of the objects seen in this image are located near the Galactic center, the features above and below the Galactic plane tend to lie closer to Earth. Image credit: NASA / JPL-Caltech.
“Everything you’d ever want to learn about the way huge black holes interact with little black holes, you can learn by studying this distribution,” said lead author Dr. Chuck Hailey, co-director of the Columbia Astrophysics Laboratory at Columbia University.
“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.”
For more than two decades, astronomers have searched unsuccessfully for evidence to support a theory that thousands of black holes surround supermassive black holes at the center of large galaxies.
“There are only about five dozen known black holes in the entire Galaxy — 100,000 light-years wide — and there are supposed to be 10,000 to 20,000 of these things in a region just six light-years wide that no one has been able to find,” Dr. Hailey said.
“Extensive fruitless searches have been made for black holes around Sagittarius A*, the nearest supermassive black hole and therefore the easiest to study. There hasn’t been much credible evidence.”
“Sagittarius 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 supermassive black hole as they lose their energy, causing them to be pulled into the vicinity of the supermassive black hole, where they are held captive by its force.”
While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary.
Scientists believe there is a heavy concentration of these isolated and mated black holes in the Galactic center, forming a density cusp which gets more crowded as distance to the supermassive black hole decreases.
In the past, failed attempts to find evidence of such a cusp have focused on looking for the bright burst of X-ray glow that sometimes occurs in black hole binaries.
“It’s an obvious way to want to look for black holes, but the Galactic center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years,” Dr. Hailey said.
To detect black hole binaries then, Dr. Hailey and his colleagues from the United States and Chile realized they would need to look for the fainter, but steadier X-rays emitted when the binaries are in an inactive state.
“It would be so easy if black hole binaries routinely gave off big bursts like neutron star binaries do, but they don’t, so we had to come up with another way to look for them,” Dr. Hailey said.
“Isolated, unmated black holes are just black — they don’t do anything. So looking for isolated black holes is not a smart way to find them either. But 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.”
The team turned to archival Chandra data to test their technique.
The study authors searched for X-ray signatures of black hole-low mass binaries in their inactive state and were able to find 12 such objects within 3 light-years of Sagittarius A*.
They 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 Sagittarius A*.
“This finding confirms a major theory and the implications are many,” Dr. 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.”
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Charles J. Hailey et al. 2018. A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy. Nature 556: 70-73; doi: 10.1038/nature25029
