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Thousands of black holes may be orbiting Milky Way center, scientists say

The black holes orbiting the galactic center would be relatively small ones, formed by collapsed stars, and seem to have migrated into the region over several billion years, the scientists said.

The findings may help astronomers better understand how the supermassive black hole at the center of the Milky Way grows: perhaps by cannibalizing smaller black holes, the researchers added.

The finding was announced this week by Michael Muno of the University of California, Los Angeles, at a meeting of the American Astronomical Society in San Diego, Calif. It was made using NASA’s Chandra X-ray Observatory, an instrument that measures emissions of X-ray radiation by stars and other celestial objects. The observations were part of an ongoing monitoring of the region around Sagittarius A*, the supermassive black hole at the center of the Milky Way.

Muno and colleagues analyzed X-ray sources near the galaxy’s center. The regions around black holes sometimes emit powerful X-rays as a result of the violent processes that occur as black holes swallow surrounding material.

Among the thousands of X-ray sources detected within 70 light years of Sgr A*, Muno and his colleagues searched for those most likely to be active black holes and neutron stars. A neutron star is another type of collapsed star that is also extremely dense, though less so than a black hole. One spoonful of neutron star material is believed to weigh as much as all the cars on Earth.

Muno and colleagues selected only the brightest sources whose X-ray output varied widely. These characteristics are believed to identify black holes and neutron stars that are swallowing material from companion stars orbiting them.

Of the seven sources that met these criteria, four are within three light years of Sagittarius A*, Muno said. This high concentration of X-ray sources near the center “implies that a huge number of black holes and neutron stars have gathered in the center of the Galaxy,” Muno said.

Mark Morris, also of UCLA and a collaborator of Muno, predicted a decade ago that a process called dynamical friction would cause stellar black holes to sink toward the center of the Galaxy. Black holes are formed as remnants of the explosions of massive stars, and have masses of about 10 suns. As black holes orbit the center of the Galaxy at a distance of several light years, they pull on surrounding stars, which pull back on the black holes.

The result is that black holes spiral inward, and the low-mass stars move out. From the estimated number of stars and black holes in the Galactic Center region, dynamical friction is expected to produce a dense swarm of 20,000 black holes within three light years of Sagittarius A*. A similar effect is at work for neutron stars, but to a lesser extent because they have a lower mass.

Once black holes are concentrated near Sagittarius A*, they will have many close encounters with normal stars there. Some of these normal stars are part of binary star systems, in which two stars circle each other.

The intense gravity of a black hole can make an ordinary star “change partners” and pair up with the black hole while ejecting its companion. This process and a similar one for neutron stars are expected to produce several hundreds of black hole and neutron star binary systems.

“If only one percent of these binary systems are X-ray active each year, they can account for the sources we see,” said Eric Pfahl of the University of Virginia in Charlottesville and a coauthor of a paper describing these results that has been submitted to the research journal Astrophysical Journal Letters.

“Although the evidence is mostly circumstantial, it makes a strong case for the existence of a large population of neutron stars and stellar-mass black holes within three light-years of the center of our Galaxy.”

The black holes and neutron stars in the cluster are expected to gradually be swallowed by the supermassive black hole, Sagittarius A*. This occurs at a rate of about one every million years, the astronomers said. At this rate, about 10,000 black holes and neutron stars would have been captured in a few billion years, adding about 3 percent to the mass of the central supermassive black hole, which is currently estimated to weigh as much as 3.7 million suns.

In the meantime, black holes will fling low-mass stars out of the central region, the researchers said. This expulsion will reduce the likelihood that normal stars will be captured by the central supermassive black hole. This may explain, they added, why the central regions of some galaxies -- including the Milky Way -- look fairly calm even though they contain a supermassive black hole.

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