Steve Roy

Marshall Space Flight Center

Huntsville, AL

Phone: 256-544-6535

Dr. Wallace Tucker

Chandra X-ray Observatory Center

Harvard-Smithsonian Center for Astrophysics

Cambridge, MA

Phone: 617-496-7998

CXC PR: 00-03

Chandra Finds a “Cool” Black Hole at the Heart of the Andromeda Galaxy

In its first look at the Andromeda Galaxy (M31), NASA’s Chandra X-ray Observatory has found that the
gas funneling into a supermassive black hole in the heart of this galaxy is a “cool” million degrees. This
unexpected result adds one more quirk to the strange behavior previously observed at the center of

A team of scientists from the Harvard-Smithsonian Center for
Astrophysics, Cambridge, Mass., reported on this observation at the 195th national meeting of the
American Astronomical Society in
Atlanta, Ga. The team is led by Drs. Stephen Murray and Michael Garcia, and includes Drs. Frank Primini,
William Forman, Christine Jones, and Ralph Kraft.

Chandra took its first X-ray picture of the Andromeda Galaxy with the Advanced CCD Imaging
Spectrometer on October 13, 1999. More than100 individual X-ray sources were seen. One of these
sources was at the previously determined position of the central supermassive black hole, which has the
mass of 30 million suns. With many X-ray emitting stars in the center of M31 there was a slight chance
that one of them might be at this position just by coincidence. The low temperature of the suspected
central source, as compared to the other sources, gave the team the clue they needed.

“When we found that what we suspected was the central object was also anomalously cool, we KNEW
we had it — one coincidence might be believable, but two was too much to ignore!” said Garcia.

While the gas falling into the central black hole is cool, it is only cool by comparison to the 100 other
X-ray sources in the Andromeda Galaxy. To be detected by an X-ray telescope, the gas must have a
temperature of more than a million degrees. The typical X-ray star in the Andromeda Galaxy has a
temperature of several tens of millions of degrees. In contrast, the temperature of the supermassive
black hole source is a few million degrees.

The Andromeda Galaxy is our nearest neighbor spiral galaxy at a distance of two million light years. It is
similar to our own Milky Way in size, shape, and also contains a supermassive black hole at the center.
This central black hole has always been a bit odd when compared to central black holes in similar
galaxies. Based on its X-ray luminosity, it is much fainter in radio waves than expected.

Such behavior, coupled with Chandra’s discovery of the low temperature gas, cannot be accommodated
by standard models developed for
supermassive black holes in galaxies like the Milky Way and Andromeda.

“The Chandra observation is telling us that an entirely different flow pattern is operating around the
Andromeda black hole,” said Dr. Eliot Quataert, of the Institute for Advanced Study, Princeton, N.J. “This
will require a different class of models than usually considered.”

One possibility is that the gas undergoes a large scale boiling motion which slows down the rate at which
gas falls into the black hole.

The best previous X-ray pictures were not sharp enough to clearly distinguish the X-ray source
associated with the black hole in the center of the Andromeda Galaxy nor did they give information
about the temperature of the source.

“A good analogy might be to say that previous X-ray images were taken with a slightly out-of-focus
black and white camera, while the Chandra image is taken with a sharp, color camera” said Murray.

Another intriguing feature of this observation is the detection of a diffuse glow that extends for a
thousand light years around the central region. It is not known if this is due to many individual sources,
or to a hot wind expanding out from the center.

“This is just a first, quick look at our nearest Milky Way analog,” Murray emphasized. “I expect that our
future pictures will lead to more exciting discoveries in the Andromeda Galaxy.”

The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and
Pennsylvania State University, University Park.

To follow Chandra’s progress, visit the Chandra site at:


NASA’s Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW, Inc.,
Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian’s Chandra X-ray
Center controls science and flight operations from Cambridge, Mass.

High resolution digital versions of the X-ray image (JPG, 300 dpi TIFF ) and other information associated
with this release are available on the Internet at:


Chandra Image of Andromeda Galaxy (M31)

This X-ray image shows the central portion of the Andromeda Galaxy. The blue dot in the center of the
image is a “cool” million degree X-ray source where a supermassive black hole with the mass of 30 million
suns is located. The X-rays are produced by matter funneling toward the black hole. Numerous other
hotter X-ray sources are also apparent. Most of these are probably due to X-ray binary systems, in
which a neutron star or black hole is in a close orbit around a normal star.

Image made with the Advanced CCD Imaging Spectrometer (ACIS)