Long ago, a giant eruption occurred in a nearby galaxy and plunged it into
turmoil. Now NASA’s Chandra X-ray Observatory has revealed the remains of
that explosion in the form of two enormous arcs of hot gas. This discovery
can help astronomers better understand the cause and effect of violent
outbursts from the vicinity of supermassive black holes in the centers of
many so-called “active” galaxies.

Scientists from the Harvard-Smithsonian Center for Astrophysics (CfA) report
that two arc-like structures of multimillion-degree gas in the galaxy
Centaurus A appear to be part of a ring 25,000 light years in diameter. The
size and location of the ring suggest that it could have been produced in a
titanic explosion that occurred about ten million years ago.

A composite image of the galaxy made with radio (red and green), optical
(yellow-orange), and X-ray data (blue) presents a stunning tableau of a
tumultuous galaxy. A broad band of dust and cold gas is bisected at an
angle by opposing jets of high-energy particles blasting away from the
supermassive black hole in the nucleus. Lying in a plane perpendicular to
the jets are the two large arcs of X-ray emitting hot gas.

“Putting all the images together was the key to understanding what Chandra
showed,” said Margarita Karovska, lead author on the paper to appear in the
September 6, 2002 of The Astrophysical Journal. “Suddenly it all clicked in,
as with a giant puzzle, and the images fit together to make a complete
picture of the galaxy geometry that was not at all apparent before.”

The team proposes that the orientation of the arcs of hot gas perpendicular
to the jet and the symmetry of the projected ring with respect to the center
of the galaxy could be evidence that the ring is the result of a giant
eruption in the nucleus of the galaxy 10 million years ago. This explosion
may have produced a galaxy-sized shock wave that has been moving outward at
speeds of a million miles per hour. The age of 10 million years for the
outburst is consistent with other optical and infrared observations that
indicate that the rate of star formation in the galaxy increased
dramatically at about that time.

Other authors have suggested that the merger of a small spiral galaxy with
Centaurus A about a hundred million years ago triggered the high-energy jets
and the ongoing violent activity in the nucleus of the galaxy. The
tremendous energy released when a galaxy is “turned on” by a collision can
have a profound influence on the subsequent evolution of the galaxy and its
neighbors. The mass of the central black hole can increase, the gas
reservoir for the next generation of stars can be expelled, and the space
between the galaxies can be enriched with heavier elements.

“Active galaxies could have played a significant role in the evolution of
galaxies in the early universe when collisions between galaxies were much
more frequent,” said Giuseppina Fabbiano, a coauthor on the paper.
“Centaurus A, at a distance of only 11 million light years, gives us a rare
opportunity to study such an active galaxy in action.”

Chandra observed Centaurus A with its High Resolution Camera instrument on
September 10, 1999, for approximately 4.7 hours. Other members of CfA
research team include Martin Elvis, Ralph Kraft, Stephen Murray, and
Fabrizio Nicastro

The HRC was built by the Smithsonian Astrophysical Observatory in Cambridge,
Mass. NASA’s Marshall Space Flight Center, Huntsville, AL, manages the
Chandra program for the Office of Space Science, Washington, DC. TRW, Inc.,
Redondo Beach, California, is the prime contractor for the spacecraft. The
Smithsonian’s Chandra X-ray Center controls science and flight operations
from Cambridge, MA.

Images associated with this release are available at: