NASA’s Chandra X-ray Observatory has provided the best X-ray image
yet of two Milky Way-like galaxies in the midst of a head-on collision.
Since all galaxies – including our own – may have undergone mergers, this
provides insight into how the Universe came to look as it does today.

Astronomers believe the mega-merger in the galaxy known as Arp 220
triggered the formation of huge numbers of new stars, sent shock waves
rumbling through intergalactic space, and could possibly lead to the
formation of a supermassive black hole in the center of the new conglomerate
galaxy. The Chandra data also suggest that merger of these two galaxies
began only 10 million years ago, a short time in astronomical terms.

“The Chandra observations show that things really get messed up when
two galaxies run into each other at full speed,” said David Clements of the
Imperial College, London, one of the team members involved in the study.
“The event affects everything from the formation of massive black holes to
the dispersal of heavy elements into the universe.”

Arp 220 is considered to be a prototype for understanding what
Conditions were like in the early Universe, when massive galaxies and
supermassive black holes were presumably formed by numerous galaxy
collisions. At a relatively nearby distance of about 250 million light
years, Arp 220 is the closest example of an “ultra-luminous” galaxy, one
that gives off a trillion times as much radiation as our Sun.

The Chandra image shows a bright central region at the waist of a
Glowing, hour-glass-shaped cloud of multimillion-degree gas. Rushing out of
the galaxy at hundreds of thousands of miles per hour, the super-heated as
forms a “superwind,” thought to be due to explosive activity generated by
the formation of hundreds of millions of new stars.

Farther out, spanning a distance of 75,000 light years, are giant
lobes of hot gas that could be galactic remnants flung into intergalactic
space by the early impact of the collision. Whether the lobes will
continue to expand into space or fall back into Arp 220 is unknown.

The center of Arp 220 is of particular interest. Chandra
observations allowed astronomers to pinpoint an X-ray source at the exact
location of the nucleus of one of the pre-merger galaxies. Another fainter
X-ray source nearby may coincide with the nucleus of the other galaxy
remnant. The X-ray power output of these point-like sources is greater than
expected for stellar black holes accreting from companion stars. The
authors suggest that these sources could be due to supermassive black holes
at the centers of the merging galaxies.

These two remnant sources are relatively weak, and provide strong
evidence to support the theory that the extraordinary luminosity of Arp 220
– about a hundred times that of our Milky Way galaxy – is due to the rapid
rate of star formation and not to an active, supermassive black hole in the

However, in a few hundred million years, this balance of power may
change. The two massive black holes could merge to produce a central
supermassive black hole. This new arrangement could cause much more gas to
fall into the central black hole, creating a power source equal to or
greater than that due to star formation.

“The unusual concentration of X-ray sources in the very center of
Arp 220 suggests that we could be observing the early stages of the creation
of a supermassive black hole and the eventual rise to power of an active
galactic nucleus,” said Jonathan McDowell of the Harvard-Smithsonian Center
for Astrophysics, Cambridge, MA, another member of the team Studying Arp

Clements and McDowell were joined on this research by an
international group of researchers from the United States, United Kingdom
and Spain. Chandra observed Arp 220 on June 24, 2000, for approximately
56,000 Seconds using the Advanced CCD Imaging Spectrometer (ACIS)

ACIS was developed for NASA by Pennsylvania State University,
University Park, Pa., and the Massachusetts Institute of Technology,
Cambridge, Mass. NASA’s Marshall Space Flight Center in Huntsville, Ala.,
manages the Chandra program, and TRW, Inc., Redondo Beach, Calif., is the
prime Contractor. The Smithsonian’s Chandra X-ray Center controls science
and flight operations from Cambridge.

Images and additional information about this result are available