Trail of Black Holes and Neutron Stars Points to Ancient Collision(Forwarded)

Steve Roy
Marshall Space Flight Center, Huntsville, AL
Phone: 256-544-6535

Megan Watzke
Chandra X-ray Observatory Center, CfA, Cambridge, MA
Phone: 617-496-7998

Science Contacts:
Lars Hernquist, , 617-496-4180
Andreas Zezas, , 617-496-7637

December 8, 2003

CXC RELEASE: 01-12

Trail of Black Holes and Neutron Stars Points to Ancient Collision

An image of an elliptical galaxy by NASA's Chandra X-ray Observatory has
revealed a trail of black holes and neutron stars stretching more than fifty
thousand light years across space. The trail of intense X-ray sources is
evidence that this apparently sedate galaxy collided with another galaxy a few
billion years ago.

"This discovery shows that X-ray observations may be the best way to identify
the ancient remains of mergers between galaxies," said Lars Hernquist of the
Harvard-Smithsonian Center for Astrophysics in Cambridge (CfA), Massachusetts,
and a coauthor on an article on the galaxy NGC 4261 in an upcoming issue of The
Astrophysical Journal Letters. "It could be a significant tool for probing the
origin of elliptical galaxies."

"From the optical and radio images, we knew something unusual was going on in
the nucleus of this galaxy, but the real surprise turned out to be on the outer
edges of the galaxy," said Andreas Zezas, also of CfA, and lead-author of the
paper on NGC 4261. "Dozens of black holes and neutron stars were strung out
across space like beads on a necklace."

The spectacular structure is thought to represent the aftermath of the
destruction of a smaller galaxy that was pulled apart by gravitational tidal
forces as it fell into NGC 4261. As the doomed galaxy fell into the larger one,
large streams of gas were pulled out into long tidal tails.

Shock waves generated as these tidal tails fell into the larger galaxy triggered
the formation of large numbers of massive stars which over the course of a few
million years evolved into neutron stars or black holes. A few of these
extremely compact objects had companion stars, and became bright X-ray sources
as gas from the companions was captured by the intense gravitational fields of
the neutron stars and black holes.

The origin of elliptical galaxies has long been a subject of intense debate
among astronomers. The currently favored view is that they are produced by
collisions between spiral galaxies. Computer simulations of galaxy collisions
support this idea, and optical evidence of tails, shells, ripples, arcs and
other structures have been interpreted as evidence for this theory. However the
optical evidence rather quickly fades into the starry background of the galaxy,
whereas the NGC 4261 X-ray observations show that the X-ray signature may linger
for hundreds of millions of years.

NGC 4261 is approximately 100 million light years away from Earth. The data for
these results were taken from the Chandra archive. NGC 4261 was originally
observed with the Advanced CCD Imaging Spectrometer on May 6, 2000. Other
members of the research team were Pepi Fabbiano and Jon Miller, both from the CfA.

NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra
program for the Office of Space Science, NASA Headquarters, Washington. Northrop
Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development
contractor for the observatory. The Smithsonian Astrophysical Observatory
controls science and flight operations from the Chandra X-ray Center in
Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2003/ngc4261/

and

http://www1.msfc.nasa.gov/NEWSROOM/n...tos03-207.html