NASA's Spitzer Spies Monster Galaxy Pileup

PureInsight | August 19, 2007

NASA Jet Propulsion Laboratory Staff

Four galaxies are slamming into each other and kicking up billions of
stars in one of the largest cosmic smash-ups ever observed.

The clashing galaxies, spotted by NASA's Spitzer Space Telescope, will
eventually merge into a single, behemoth galaxy up to 10 times as
massive as our own Milky Way. This rare sighting provides an
unprecedented look at how the most massive galaxies in the universe

"Most of the galaxy mergers we already knew about are like compact cars
crashing together," said Kenneth Rines of the Harvard-Smithsonian
Center for Astrophysics, Cambridge, Mass. "What we have here is like
four sand trucks smashing together, flinging sand everywhere." Rines is
lead author of a new paper accepted for publication in Astrophysical
Journal Letters.

One of the
biggest galaxy collisions ever observed is taking place at the center
of this image. The four yellow blobs in the middle are large galaxies
that have begun to tangle and ultimately merge into a single gargantuan
galaxy. The yellowish cloud around the colliding galaxies contains
billions of stars tossed out during the messy encounter. Other galaxies
and stars appear in yellow and orange hues.

NASA's Spitzer
Space Telescope spotted the four-way collision, or merger, in a giant
cluster of galaxies, called CL0958+4702, located nearly five billion
light-years away. The dots in the picture are a combination of galaxies
in the cluster; background galaxies located behind the cluster; and
foreground stars in our own Milky Way galaxy.

Infrared data
from Spitzer are colored red in this picture, while visible-light data
from a telescope known as WIYN are green. Areas where green and red
overlap appear orange or yellow. Since most galaxies in the cluster
contain old stars that are visible to Spitzer and WIYN, those galaxies
appear orange.

[Image credit: NASA/JPL-Caltech/CXO/WIYN/Harvard-Smithsonian CfA]

Collisions, or mergers, between galaxies are common in the universe.
Gravity causes some galaxies that are close together to tangle and
ultimately unite over a period of millions of years. Though stars in
merging galaxies are tossed around like sand, they have a lot of space
between them and survive the ride. Our Milky Way galaxy will team up
with the Andromeda galaxy in five billion years.

Mergers between one big galaxy and several small ones, called minor
mergers, are well documented. For example, one of the most elaborate
known minor mergers is taking place in the Spiderweb galaxy -- a
massive galaxy that is catching dozens of small ones in its "web" of
gravity. Astronomers have also witnessed "major" mergers among pairs of
galaxies that are similar in size. But no major mergers between
multiple hefty galaxies - the big rigs of the galaxy world - have been
seen until now.

The new quadruple merger was discovered serendipitously during a
Spitzer survey of a distant cluster of galaxies, called CL0958+4702,
located nearly five billion light-years away. The infrared telescope
first spotted an unusually large fan-shaped plume of light coming out
of a gathering of four blob-shaped, or elliptical, galaxies. Three of
the galaxies are about the size of the Milky Way, while the fourth is
three times as big.

This artist's
concept shows what the night sky might look like from a hypothetical
planet around a star tossed out of an ongoing four-way collision
between big galaxies (yellow blobs). NASA's Spitzer Space Telescope
spotted this "quadruple merger" of galaxies within a larger cluster of
galaxies located nearly 5 billion light-years away.

Though the
galaxies appear intact, gravitational disturbances have caused them to
stretch and twist, flinging billions of stars into space - nearly three
times as many stars as are in our Milky Way galaxy. The tossed stars
are visible in the large plume emanating from the central, largest
galaxy. If any of these stars have planets, their night skies would be
filled with the monstrous merger, along with other galaxies in the
cluster (smaller, bluish blobs).

This cosmic
smash-up is the largest known merger between galaxies of a similar
size. While three of the galaxies are about the size of our Milky Way
galaxy, the fourth (center of image) is three times as big. All four of
the galaxies, as well as most other galaxies in the huge cluster, are
blob-shaped ellipticals instead of spirals like the Milky Way.

Ultimately, in
about one hundred million years or so, the four galaxies will unite
into one. About half of the stars kicked out during the merger will
fall back and join the new galaxy, making it one of the biggest
galaxies in the universe.  

[Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA]

Further analysis of the plume revealed it is made up of billions of
older stars flung out and abandoned in an ongoing clash. About half of
the stars in the plume will later fall back into the galaxies. "When
this merger is complete, this will be one of the biggest galaxies in
the universe," said Rines.

The Spitzer observations also show that the new merger lacks gas.
Theorists predict that massive galaxies grow in a variety of ways,
including gas-rich and gas-poor mergers. In gas-rich mergers, the
galaxies are soaked with gas that ignites to form new stars. Gas-poor
mergers lack gas, so no new stars are formed. Spitzer found only old
stars in the quadruple encounter.

"The Spitzer data show that these major mergers are gas-poor, unlike
most mergers we know about," said Rines. "The data also represent the
best evidence that the biggest galaxies in the universe formed fairly
recently through major mergers."

Some of the stars tossed out in the monstrous merger will live in
isolated areas outside the borders of any galaxies. Such abandoned
stars could theoretically have planets. If so, the planets' night skies
would be quite different from our own, with fewer stars and more
visible galaxies.

In addition to Spitzer, Rines and his team used a telescope formerly
known as the Multiple Mirror Telescope and now called MMT near Tucson,
Ariz., to confirm that the four galaxies are intertwined, and NASA's
Chandra X-ray Observatory to weigh the mass of the giant cluster of
galaxies in which the merger was discovered. Both Spitzer and a
telescope known as WIYN at Kitt Peak, also near Tucson, Ariz., were
used to study the plume. WIYN is named after the University of
Wisconsin, Indiana University, Yale University and the National Optical
Astronomy Observatory, which own and operate the telescope.

Other authors of this paper include Rose Finn of Siena College,
Loudonville, N.Y.; and Alexey Vikhlinin of the Harvard-Smithsonian
Center for Astrophysics.


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