NASA’s Chandra X-ray Observatory has revealed the
aftermath of a titanic explosion that wracked the elliptical
galaxy known as NGC 4636. This eruption could be the latest
episode in a cycle of violence triggered by gas falling into a
central massive black hole.

Chandra’s images of NGC 4636 show spectacular symmetric arms,
or arcs, of hot gas extending 25,000 light-years into a huge
cloud of multimillion-degree Celsius gas that envelopes the
galaxy. At a temperature of 10 million degrees, the arms are
30 percent hotter than the surrounding gas cloud.

“The temperature jump, together with the symmetry and scale of
the arms, suggests that we are observing the effects of a
tremendous outburst that occurred in the center of the
galaxy,” said Christine Jones of the Harvard-Smithsonian
Center for Astrophysics (CfA) in Cambridge, Mass., lead author
of a paper on these observations scheduled for publication in
Astrophysical Journal Letters. “The energy of this explosion
would be the equivalent of several hundred thousand

The arms appear to be the leading edges of a galaxy-sized
shock wave that is racing outward at 700 kilometers per
second, or 1.6 million miles per hour. At this speed, it would
take 3 million years for the structures to attain their
present size. Cavities detected in the hot gas cloud to the
east and west of the center of the galaxy support the shock-
wave explanation.

The authors suggest that the explosion is part of a majestic
cosmic feedback process that keeps the galaxy in a state of
turmoil. Over a period of a few million years, a hot gas cloud
that envelops the stars in the galaxy cools and falls inward
toward a central, massive black hole. The feeding of the black
hole by the infalling material leads to an explosion that
heats the hot gaseous envelope, starting the cycle anew.

This feedback cycle may explain one puzzling feature of the
galaxy — the lack of a strong radio source of the type that
is usually observed in connection with galactic outbursts.

“It may be that we are seeing an early stage of the cycle
before the radio source has turned on,” said team member
William Forman, also of the Harvard-Smithsonian Center for
Astrophysics. “Or, it could be a new type of outburst that is
not accompanied by strong radio emission.”

Other members of the team included Alexey Vikhlinin, Maxim
Markevitch, Laurence David, Aryeh Warmflash, all of the CfA,
and Paul Nulsen of the University of Wollongong in Australia.
Chandra observed NGC 4636, an elliptical galaxy in the
constellation Virgo some 50 million light-years from Earth,
with the Advanced CCD Imaging Spectrometer (ACIS) on Dec. 4-5,
1999, for 11,000 seconds and Jan. 26-27, 2000, for 53,000
seconds. These observations were part of a program led by
Richard Mushotzky of NASA’s Goddard Space Flight Center,
Greenbelt, Md., to study X-ray emission from elliptical

The ACIS instrument 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 on the World
Wide Web at: