The Andromeda
Drain
The Chandra X-ray Observatory has spied a
peculiar black hole at the center of the Andromeda galaxy.
January 26, 2000 -- Just 2 million light years away lies
Andromeda, a big beautiful spiral nebula that is the nearest
major galaxy to the Milky Way. It's so nearby, in fact, that
you can see it with the naked eye on a dark night as a faint
fuzzy blob in the northern sky. Andromeda is probably the most
thoroughly studied galaxy in the sky -- second only to the
Milky Way itself. Much of our own galaxy is hidden from view
by dense clouds of interstellar dust. Andromeda gives
astronomers a chance to look at a young spiral galaxy from the
outside, to see the complete picture that's hidden from us in
the Milky Way.
Above: The diffuse light from the Andromeda
galaxy is caused by hundreds of billions of stars. The several
distinct stars that surround Andromeda's image are actually
stars in our Galaxy that are well in front of the background
object. Andromeda is frequently referred to as M31 since it is
the 31st object on Messier's list of diffuse sky objects.
When NASA's Chandra X-ray Observatory was
launched in 1999, many astronomers were understandably anxious
to view the first high resolution X-ray pictures of the Milky
Way's nearest galactic neighbor. Earlier
data from the Hubble Space Telescope had revealed a black
hole candidate in Andromeda's nucleus weighing in at 30
million solar masses. If there really was a supermassive black
hole there, then there should be an X-ray source betraying its
presence.
Why does a black hole emit X-rays?
It's not really the black hole itself that
shines as an X-ray source. Stars and interstellar gas in the
vicinity of the hole can be trapped by its powerful gravity.
As matter spirals in toward the hole, much like water flowing
down a drain, it heats up to millions of degrees and begins to
glow in X-radiation. The disk of material that surrounds a
black hole is called an accretion
disk.
Chandra took its first X-ray picture of
Andromeda on October 13, 1999, and there were more than 100
individual X-ray sources in the image. Most of them are
thought to be binary star systems, but one was located
precisely at the galactic center just where the black hole
ought to be. This greatly strengthens the case for an awesome
concentration of mass at Andromeda's center.
Left: This false-color X-ray picture from the Chandra X-ray
Observatory shows a number of X-ray sources, most likely
binary stars, within Andromeda's central region as yellowish
dots. The blue source located right at the galaxy's center is
coincident with the position of a suspected massive black
hole. [more
information]
The black hole candidate in Andromeda is big
-- 30 million times more massive than our Sun -- but it's not
a record setter. Some active galaxies appear to harbor black
holes in their nucleus that register between 100 million and a
billion solar masses.
Andromeda's black hole appears to be
remarkable for a different reason. Data from Chandra's
advanced spectrometer showed that the temperature of its
accretion disk was just one million degrees. By everyday
standards on Earth, that's hot. But to X-ray astronomers it's
very cool. Matter doesn't even register on an X-ray telescope
until its temperature reaches about one million degrees. For
comparison, the other sources in the Chandra image register a
sizzling 10 million degrees. They are probably binary star
systems in which a normal star orbits a neutron star or a
small black hole. The normal star feeds matter to an accretion
disk around its dense companion, resulting in X-ray emission
from the hot disk. These systems weigh in at just a few to a
few tens of solar masses. Theorists expected the accretion
disk around the central massive black hole to be at least as
hot and energetic as these lightweight systems.
"The Chandra observation is telling us
that an entirely different flow pattern [must be] 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 we
usually consider."
Right: The Electromagnetic Spectrum. The wavelength of
radiation produced by an object is usually related to its
temperature. [learn
more about x-rays]
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 stars near Andromeda's
center from the black hole, nor did they give information
about the temperature of the source. Chandra has changed all
that.
"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 Stephen Murray, a member of
the Harvard team studying Andromeda. "This is just a
first, quick look at our nearest Milky Way analog. I expect
that our future pictures will lead to more exciting
discoveries in the Andromeda Galaxy."
The Advanced CCD Imaging Spectrometer, which
was used to detect the central black hole in Andromeda, was
built for NASA by the Massachusetts Institute of Technology,
Cambridge, and Pennsylvania State University, University Park.
Related Links:
Chandra
home page -from Harvard
another
Chandra home page -from NASA
Black
Holes -a tutorial about black holes and accretion disks
X-Rays
- Another Form of Light - the basics of X-rays from the
Chandra home page at Harvard
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