Astronomers Go Behind the Milky Way to Solve X-ray Mystery
Astronomers using the Chandra X-ray Observatory have peered behind
the Milky Way galaxy for the first time, through layers of dust and
gas that stretch out for 30,000 light years. They discovered 36
bright, distant galaxies lurking back there, and they found that the
source of the Milky Way's X-ray emission is hot, diffuse gas.
Chandra's image of the "zone of avoidance." The diffuse blue emission is due to hot (ten million degree Celsius)
gas concentrated along the plane of the Galaxy. Most of the pink and
red objects in this image are believed to be to be active stars in our
Milky Way galaxy. The blue objects are considered to be distant
galaxies because of their x-ray properties.
(Credit: NASA/GSFC/Ebisawa et al.)
The findings, published in the online version of Science on August
10, settle a long-standing mystery about the source of the X-ray
emission from the galactic plane and have broad implications for
theories of how our galaxy has evolved.
Since the discovery in the mid-1970s of X-ray radiation in the Milky
Way's galactic plane with the Uhuru satellite, scientists have
debated whether this ubiquitous emission is diffuse light or from
individual stars. Armed with Chandra, an international team led Dr.
Ken Ebisawa of NASA Goddard Space Flight Center zoomed in on a tiny
region of the galactic plane to investigate.
"The point sources we saw in the galactic plane were actually active
galaxies with bright cores millions of light years behind our
galaxy," said Ebisawa. "The number of these sources is consistent
with the expected number of extragalactic sources in the background
sky. We saw few additional point sources within our Galaxy."
Because Ebisawa and his colleagues were able to identify these
objects as being well beyond the galactic plane, they were able
to determine that the X-ray glow from the galactic plane comes
not from individual sources, but from the hot diffuse gas.
Light from the 36 newly discovered galaxies passes through our own
Galaxy on its journey towards Earth. As the X-ray light passes
through the Milky Way, it is affected by all that it passes through,
thereby allowing astronomers to measure the distribution and physical
condition of matter in our Galaxy.
Detection of diffuse X-rays emanating from the Galactic plane
indicates the presence of extremely hot, previously unknown, plasma
gas. This gas is 100 times more energetic than any known existing
gas in the interstellar medium, Ebisawa said.
Gas this hot would escape the gravitational confines of the Milky Way
galaxy under normal circumstances. The fact that it still lingers
within the Galactic plane is the next mystery to solve. Ebisawa
suggests that magnetic fields may act to confine the heat and gas.
Theorists, however, will wrestle with the Chandra observation's
implications in months to come.
Some astronomers have proposed that white dwarfs -- stars like our
Sun that have collapsed -- could have contributed to the Galactic
plane X-ray emission. The Chandra observation rules out this
possibility and implies there are fewer white dwarfs in our galaxy
than previously thought.
The Chandra observation, conducted in February 2000, lasted 28 hours.
The team observed what was known to be a "blank" region of the
galactic plane in the constellation Scutum where an earlier Japanese
X-ray satellite called ASCA found no individual X-ray sources.
The observation marks the first time astronomers have penetrated the
heart of the so-called "zone of avoidance" -- a region of space
behind which no observation has ever been taken. Thick dust and gas
in the spiral arms of the Milky Way galaxy block out most forms of
radiation. Thus, from our vantage point on the outer edge of the
galaxy, we cannot see anything beyond the other side of our galaxy.
X rays, however, can penetrate this dust and gas. Chandra, and now
the European XMM-Newton satellite, are at long last beginning to
collect light from behind our galaxy.
Participating in the Chandra observation and Science article are
Yoshitomo Maeda of Pennsylvania State University; Hidehiro Kaneda of
the Institute of Space and Astronautical Science in Japan; and Shogeo
Yamauchi of Iwate University in Japan. Ebisawa joins NASA Goddard
through the University Space Research Association.