A monster galaxy pileup

A monster galaxy pileup

NEW HAVEN, Conn. - 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 and
the WIYN 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 form.

"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,
who was a Mead postdoctoral fellow at Yale from 2003-6 when much of
this work was done, is lead author of a paper accepted for publication
in Astrophysical Journal Letters.

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
survey of a distant cluster of galaxies, called CL0958+4702, located
nearly five billion light-years away. The telescopes 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.

"The colors from the WIYN and Spitzer data show that the stars are
old, but the higher resolution WIYN images show that the light from
the disrupted galaxy does not have small-scale structure but is
instead smoothly distributed telling us that the galaxies involved in
the merger are elliptical rather than spiral galaxies," said Jeffrey
Kenney, professor and chair of Astronomy at Yale.

According to Kenney, WIYN (named for it¹s joint ownership by the
University of Wisconsin, Indiana University, Yale University, and the
National Optical Astronomy Observatory) is one of the best imaging
telescopes in the world. "The sharpness of the WIYN images helps show
that we are in fact seeing a merger, and what type of galaxies have
merged."

"The WIYN telescope provides some of the sharpest images possible from
ground-based telescopes. The WIYN images show that the four galaxies
have well-defined cores that have held together during the merger,
much like egg yolks stay together longer than egg whites if you
"merge" them in a mixing bowl," said Rines.

Further analysis of the plume revealed that 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 and WIYN, 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 WIYN, also near Tucson, Ariz., were used to study the plume.

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

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the
Spitzer Space Telescope mission for NASA's Science Mission
Directorate, Washington. Science operations are conducted at the
Spitzer Science Center at the California Institute of Technology, also
in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared array
camera was built by NASA's Goddard Space Flight Center, Greenbelt, Md.
The instrument's principal investigator is Giovanni Fazio of the
Harvard-Smithsonian Center for Astrophysics.

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