Through layers of gas and dust that stretch for more than 30,000
light years, astronomers using NASA’s Chandra X-ray Observatory have taken a
long, hard look at the plane of the Milky Way galaxy and found that its
X-ray glow comes from hot and diffuse gas. The findings, published in the
Aug. 10 issue of Science, help to settle a long-standing mystery about the
source of the X-ray emission from the galactic plane.
Scientists have debated whether the Milky Way plane’s X-ray emission
was diffuse light or from individual stars. Armed with Chandra, an
international team led Dr. Ken Ebisawa of NASA’s Goddard Space Flight
Center, Greenbelt, Md., zoomed in on a tiny region of the galactic plane in
the constellation Scutum.
“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.”
The observation marks the deepest X-ray look at the so-called “zone
of avoidance” — a region of space behind which no optical observation has
ever been taken because thick dust and gas in the spiral arms of the Milky
Way galaxy block out visible radiation. Infrared, radio, and X-rays,
however, can penetrate this dust and gas. Detection of diffuse X-rays
emanating from the galactic plane, what we call the “Milky Way” in visible
light, indicates the presence of plasma gas with temperatures of tens of
millions of degrees Celsius.
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. One possibility, suggested
by Ebisawa is that hot plasma may be confined to the Milky Way by magnetic
fields.
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 where the Japanese X-ray satellite ASCA had previously
observed but found no individual X-ray sources.
The team also discovered 36 bright distant galaxies lurking in the
background of this section of the galactic plane, while the foreground was
devoid of stars or other individual objects emitting X-rays. Chandra, and
now the European XMM-Newton satellite, are at long last beginning to collect
light from behind our galaxy. X-ray radiation from the 36 newly discovered
galaxies passes through the Milky Way on its journey towards Earth. This
light, therefore, carries the imprint of all that it passes through and will
allow astronomers to measure the distribution and physical condition of
matter in 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 Shigeo Yamauchi
of Iwate University in Japan.
Chandra observed the galactic plane with its Advanced CCD Imaging
Spectrometer (ACIS) instrument, which was developed for NASA by Pennsylvania
State University, University Park, and Massachusetts Institute of
Technology, Cambridge. NASA’s Marshall Space Flight Center in Huntsville,
Ala., manages the Chandra program, and 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.
Images associated with this release are available at:
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