They're Gathered Near The Galactic Center
A swarm of 10,000 or more black
holes may be orbiting the Milky Way's supermassive black hole,
according to new results from NASA's Chandra X-ray Observatory.
This may represent the highest concentration of black holes
anywhere in the Galaxy.
These relatively small, stellar-mass black holes, along with
neutron stars, appear to have migrated into the Galactic Center
over the course of several billion years. This dense stellar
graveyard has been predicted for years, and this represents the
best evidence to date of its existence. These data may also help
astronomers better understand how the supermassive black hole at
the center of the Milky Way feeds.
The discovery was made as part of Chandra's ongoing program of
monitoring the region around Sagittarius A*, or Sgr A*, the
supermassive black hole at the center of the Milky Way. It was
announced today by Michael Muno of the University of California,
Los Angeles, at a meeting of the American Astronomical Society in
San Diego, CA.
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 by selecting only
the brightest sources that also exhibited large variations in their
X-ray output. These characteristics identify black holes and
neutron stars that are in binary star systems and are pulling
matter from nearby companion stars. Of the seven sources that met
these criteria, four are within three light years of Sgr A*.
"Although the region around Sgr A* is crowded with stars, we
expected that there was only a 20 percent chance that we would find
even one X-ray binary within a three-light-year radius," said Muno.
"The observed high concentration of these sources implies that a
huge number of black holes and neutron stars have gathered in the
center of the Galaxy."
Mark Morris, also of UCLA and a coauthor on the present work,
had 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 net effect of this gravitational action and reaction is to
decelerate the black holes and speed up the surrounding stars. The
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, this process is expected to produce a dense swarm of
20,000 black holes within three light years of Sgr 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 Sgr A*, they will have
numerous close encounters with normal stars there, some of which
are in binary star systems. The intense gravity of a black hole can
induce an ordinary star to "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 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, Sgr A*, at a
rate of about one every million years. At this rate, about 10,000
black holes and neutron stars would have been captured in a few
billion years, adding about three percent to the mass of the
central supermassive black hole, which is currently estimated to
contain the mass of 3.7 million suns.
In the meantime, the acceleration of low-mass stars by black
holes will eject low-mass stars from the central region. This
expulsion will reduce the likelihood that normal stars will be
captured by the central supermassive black hole. This may explain
why the central regions of some galaxies, including the Milky Way,
are fairly quiet even though they contain a supermassive black
hole.
The region analyzed in this research near Sgr A* has been
observed 16 times between 1999 and 2004 using Chandra's Advanced
CCD Imaging Spectrometer instrument. Other members of the research
team include Frederick K. Baganoff, Massachusetts Institute of
Technology, Niel Brandt, Penn State, Andrea Ghez and Jessica Lu,
UCLA.
NASA's Marshall Space Flight Center, Huntsville, AL, manages the
Chandra program for NASA's Science Mission Directorate, Washington.
The Smithsonian Astrophysical Observatory controls science and
flight operations from the Chandra X-ray Center in Cambridge,
MA.