RELEASE: 00-310

Using NASA’s Chandra X-ray Observatory, astronomers have found a
giant football-shaped cavity within X-ray emitting hot gas surrounding the
galaxy Cygnus A. The cavity in the hot gas has been created by two powerful
jets emitted from the central black hole region in the nucleus of Cygnus A.

Hot gas is steadily being piled up around the cavity as it
continuously expands, creating a bright rim of X-ray emission. The jets
themselves terminate in radio and X-ray emitting “hot spots” some 300,000
light years from the center of the galaxy. These results are being
presented to the High Energy Astrophysics Division of the American
Astronomical Society meeting in Honolulu, Hawaii, by Andrew S. Wilson and
Andrew J. Young of the University of Maryland and Patrick L. Shopbell of the
California Institute of Technology.

“This is a spectacular cavity, which is inflated by jets and
completely surrounds the Cygnus A galaxy,” said Dr. Wilson, who is professor
of astronomy at the University of Maryland. “We are witnessing a battle
between the gravity of the Cygnus A galaxy, which is trying to pull the hot
gas inwards, and the pressure of material created by the jets, which is
trying to push the hot gas outwards.”

Cygnus A has long been famous as the brightest radio source in the
sky. It is the nearest powerful radio galaxy. The Chandra X-ray image, which
was taken with the Advanced CCD Imaging Spectrometer (ACIS), shows the
cavity surrounded by a vast sea of extremely hot gas. The elongated oval
shape comes from the force of the outwardly moving jets as they push through
the hot gas. Bright bands around the “equator of the football” are also
visible, and this may be evidence of material swirling toward the central
black hole.

Without the jets, an X-ray image of Cygnus A, which is about 700
million light years from Earth, would appear as a more or less spherical
region — about 2 million light years across — of hot gas slowly falling
into the Cygnus A galaxy. However, the two jets powered by the nuclear black
hole in this galaxy push this gas outward, like a balloon being inflated by
a tank of gas.

Cygnus A is not alone in its galactic neighborhood, but is a member
of a large cluster containing many galaxies. Extremely hot — tens of
millions of degrees Celsius — gas is spread between the galaxies. Although
it has a very low density, this gas provides enough resistance to slow down
the outward advancement of the particle jets from Cygnus A. At the ends of
the jets, astronomers find bright areas of radio and X-ray emission known as
“hot spots.” Scientists believe that fast atomic particles and magnetic
fields from the jets spill out into the region, providing pressure that
continuously inflates the cavity.

In a paper accepted by the Astrophysical Journal Letters, Wilson,
Young and Shopbell discuss how the Chandra observations resolve a
long-standing puzzle about the hot spots at the ends of the jets. By
analyzing the X-ray emission of the hot spots, the astronomers have measured
the strength of the magnetic field associated with them.

“The radio data themselves cannot determine the strength of the
magnetic field, a limitation that has inhibited progress in our
understanding of cosmic radio sources for 50 years,” said Wilson.
“Combination of the Chandra X-ray and the radio data allows a quite precise
measurement of the field strength.”

The Chandra observation of Cygnus A was made with the ACIS on May 21
for over nine hours.

The ACIS X-ray camera was developed for NASA by Pennsylvania State
University and Massachusetts Institute of Technology of Cambridge, Mass.
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. This research was supported by the Chandra
project at the NASA Marshall Space Flight Center.

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