Astronomers Spot Rare Arc From Hefty Galaxy Cluster

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Donna Weaver / Ray Villard 410-338-4493 / 338-4514
Space Telescope Science Institute, Baltimore, Md.
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News release: 2012-187 June 26, 2012

Astronomers Spot Rare Arc From Hefty Galaxy Cluster

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-187&cid=release_2012-187

PASADENA, Calif. -- Seeing is believing, except when you don't believe what you see.
Astronomers using NASA's Hubble Space Telescope have found a puzzling arc of light behind
an extremely massive cluster of galaxies residing 10 billion light-years away. The galactic
grouping, discovered by NASA's Spitzer Space Telescope, was observed as it existed when the
universe was roughly a quarter of its current age of 13.7 billion years.

The giant arc is the stretched shape of a more distant galaxy whose light is distorted by the
monster cluster's powerful gravity, an effect called gravitational lensing. The trouble is, the arc
shouldn't exist.

"When I first saw it, I kept staring at it, thinking it would go away," said study leader Anthony
Gonzalez of the University of Florida in Gainesville, whose team includes researchers from
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "According to a statistical analysis, arcs
should be extremely rare at that distance. At that early epoch, the expectation is that there are not
enough galaxies behind the cluster bright enough to be seen, even if they were 'lensed,' or
distorted by the cluster. The other problem is that galaxy clusters become less massive the further
back in time you go. So it's more difficult to find a cluster with enough mass to be a good lens
for gravitationally bending the light from a distant galaxy."

Galaxy clusters are collections of hundreds to thousands of galaxies bound together by gravity.
They are the most massive structures in our universe. Astronomers frequently study galaxy
clusters to look for faraway, magnified galaxies behind them that would otherwise be too dim to
see with telescopes. Many such gravitationally lensed galaxies have been found behind galaxy
clusters closer to Earth.

The surprise in this Hubble observation is spotting a galaxy lensed by an extremely distant
cluster. Dubbed IDCS J1426.5+3508, the cluster is the most massive found at that epoch,
weighing as much as 500 trillion suns. It is 5 to 10 times larger than other clusters found at such
an early time in the history of the universe. The team spotted the cluster in a search using
NASA's Spitzer Space Telescope in combination with archival optical images taken as part of
the National Optical Astronomy Observatory's Deep Wide Field Survey at the Kitt Peak National
Observatory, Tucson, Ariz. The combined images allowed them to see the cluster as a grouping
of very red galaxies, indicating they are far away.

This unique system constitutes the most distant cluster known to "host" a giant gravitationally
lensed arc. Finding this ancient gravitational arc may yield insight into how, during the first
moments after the Big Bang, conditions were set up for the growth of hefty clusters in the early
universe.

The arc was spotted in optical images of the cluster taken in 2010 by Hubble's Advanced Camera
for Surveys. The infrared capabilities of Hubble's Wide Field Camera 3 helped provide a precise
distance, confirming it to be one of the farthest clusters yet discovered.

Once the astronomers determined the cluster's distance, they used Hubble, the Combined Array
for Research in Millimeter-wave Astronomy (CARMA) radio telescope, and NASA's Chandra
X-ray Observatory to independently show that the galactic grouping is extremely massive.

"The chance of finding such a gigantic cluster so early in the universe was less than one percent
in the small area we surveyed," said team member Mark Brodwin of the University of Missouri-
Kansas City. "It shares an evolutionary path with some of the most massive clusters we see
today, including the Coma cluster and the recently discovered El Gordo cluster."

An analysis of the arc revealed that the lensed object is a star-forming galaxy that existed 10
billion to 13 billion years ago. The team hopes to use Hubble again to obtain a more accurate
distance to the lensed galaxy.

The team's results are described in three papers, which will appear online today and will be
published in the July 10, 2012 issue of The Astrophysical Journal. Gonzalez is the first author on
one of the papers; Brodwin, on another; and Adam Stanford of the University of California at
Davis, on the third. Daniel Stern and Peter Eisenhardt of JPL are co-authors on all three papers.

JPL 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 in Pasadena. Data are archived at the Infrared Science Archive housed at
the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For
more information about Spitzer, visit http://spitzer.caltech.edu and http://www.nasa.gov/spitzer .

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