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
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
(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
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
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
forces as it fell into NGC 4261. As the doomed galaxy fell into the
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.

The origin of elliptical galaxies has long been a subject of intense
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

NGC 4261 is approximately 100 million light years away from Earth. The
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
both from the CfA.

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

Additional information and images are available at: