NASA'S Chandra Turns up Black Hole Bonanza in Galaxy Next Door

June 12, 2013

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
617-496-7998
mwatzke@cfa.harvard.edu


RELEASE: 13-181

NASA'S CHANDRA TURNS UP BLACK HOLE BONANZA IN GALAXY NEXT DOOR

WASHINGTON -- Using data from NASA's Chandra X-ray Observatory,
astronomers have discovered an unprecedented bonanza of black holes
in the Andromeda Galaxy, one of the nearest galaxies to the Milky
Way.

Using more than 150 Chandra observations, spread over 13 years,
researchers identified 26 black hole candidates, the largest number
to date, in a galaxy outside our own. Many consider Andromeda to be a
sister galaxy to the Milky Way. The two ultimately will collide,
several billion years from now.

"While we are excited to find so many black holes in Andromeda, we
think it's just the tip of the iceberg," said Robin Barnard of
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge,
Mass., and lead author of a new paper describing these results. "Most
black holes won't have close companions and will be invisible to us."


The black hole candidates belong to the stellar mass category, meaning
they formed in the death throes of very massive stars and typically
have masses five to 10 times that of our sun. Astronomers can detect
these otherwise invisible objects as material is pulled from a
companion star and heated up to produce radiation before it
disappears into the black hole.

The first step in identifying these black holes was to make sure they
were stellar mass systems in the Andromeda Galaxy itself, rather than
supermassive black holes at the hearts of more distant galaxies. To
do this, the researchers used a new technique that draws on
information about the brightness and variability of the X-ray sources
in the Chandra data. In short, the stellar mass systems change much
more quickly than the supermassive black holes.

To classify those Andromeda systems as black holes, astronomers
observed that these X-ray sources had special characteristics: that
is, they were brighter than a certain high level of X-rays and also
had a particular X-ray color. Sources containing neutron stars, the
dense cores of dead stars that would be the alternate explanation for
these observations, do not show both of these features
simultaneously. But sources containing black holes do.

The European Space Agency's XMM-Newton X-ray observatory added crucial
support for this work by providing X-ray spectra, the distribution of
X-rays with energy, for some of the black hole candidates. The
spectra are important information that helps determine the nature of
these objects.

"By observing in snapshots covering more than a dozen years, we are
able to build up a uniquely useful view of M31," said co-author
Michael Garcia, also of CfA. "The resulting very long exposure allows
us to test if individual sources are black holes or neutron stars."

The research group previously identified nine black hole candidates
within the region covered by the Chandra data, and the present
results increase the total to 35. Eight of these are associated with
globular clusters, the ancient concentrations of stars distributed in
a spherical pattern about the center of the galaxy. This also
differentiates Andromeda from the Milky Way as astronomers have yet
to find a similar black hole in one of the Milky Way's globular
clusters.

Seven of these black hole candidates are within 1,000 light-years of
the Andromeda Galaxy's center. That is more than the number of black
hole candidates with similar properties located near the center of
our own galaxy. This is not a surprise to astronomers because the
bulge of stars in the middle of Andromeda is bigger, allowing more
black holes to form.

"When it comes to finding black holes in the central region of a
galaxy, it is indeed the case where bigger is better," said co-author
Stephen Murray of Johns Hopkins University and CfA. "In the case of
Andromeda we have a bigger bulge and a bigger supermassive black hole
than in the Milky Way, so we expect more smaller black holes are made
there as well."

This new work confirms predictions made earlier in the Chandra mission
about the properties of X-ray sources near the center of M31. Earlier
research by Rasmus Voss and Marat Gilfanov of the Max Planck
Institute for Astrophysics in Garching, Germany, used Chandra to show
there was an unusually large number of X-ray sources near the center
of M31. They predicted most of these extra X-ray sources would
contain black holes that had encountered and captured low mass stars.
This new detection of seven black hole candidates close to the center
of M31 gives strong support to these claims.

"We are particularly excited to see so many black hole candidates this
close to the center, because we expected to see them and have been
searching for years," said Barnard.

These results will be published in the June 20 issue of The
Astrophysical Journal. Many of the Andromeda observations were made
within Chandra's Guaranteed Time Observer program.

NASA's Marshall Space Flight Center in Huntsville, Ala., manages the
Chandra Program for NASA's Science Mission Directorate in Washington.
The Smithsonian Astrophysical Observatory controls Chandra's science
and flight operations from Cambridge, Mass.

For Chandra images, multimedia and related materials, visit:

���http://www.nasa.gov/chandra

For an additional interactive image, podcast, and video on the
finding, visit:

http://chandra.si.edu


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