NEW BRUNSWICK,
N.J. –  Rutgers astrophysicists and their collaborators have discovered an exceptional galaxy cluster, the largest seen in the
distant universe, using NASA's Chandra X-ray Observatory
and the National Science Foundation-funded Atacama Cosmology Telescope
(ACT) in Chile.

Officially known as ACT-CL J0102-4915, the
galaxy cluster has been nicknamed "El Gordo" ("the big one" or "the fat
one" in Spanish) by the researchers who discovered it. The name, in a
nod to the Chilean connection, describes just one of the remarkable
qualities of the cluster, which is located more than 7 billion light
years from Earth. This large distance means it is being observed at a
young age.

"This cluster is the most massive, the hottest, and
gives off the most X-rays of any known cluster at this distance or
beyond," said Felipe Menanteau, study leader and research scientist in the Department of Physics and Astronomy, School of Arts and Sciences, at Rutgers.

A composite image shows El Gordo in X-ray light from NASA's Chandra X-ray Observatory in blue, along with optical data from the European Southern Observatory's Very Large Telescope (VLT) in red, green, and blue, and infrared emission from the NASA's Spitzer Space Telescope in red and orange. X-ray data from Chandra reveal a distinct cometary appearance of El Gordo, including two 'tails' extending to the upper right of the image.  Along with the VLT's optical data, this shows that El Gordo is, in fact, the site of two galaxy clusters running into one another at several million miles per hour.

Galaxy clusters, the largest objects
in the universe that are held together by gravity, form through the
merger of smaller groups or sub-clusters of galaxies. Because the
formation process depends on the amount of dark matter and dark energy
in the universe, clusters can be used to study these mysterious
phenomena.

Dark matter is material that can be inferred to
exist through its gravitational effects, but does not emit and absorb
detectable amounts of light. Dark energy is a hypothetical form of
energy that permeates all space and exerts a negative pressure that
causes the universe to expand at an ever-increasing rate.

"Gigantic galaxy clusters like this are just what we were aiming to
find," said team member Jack Hughes, professor in the Department of Physics and Astronomy at Rutgers. "We want to see if
we can understand how these extreme objects form using the best models
of cosmology that are currently available."

Although a cluster
of El Gordo's size and distance is extremely rare, it is likely that its
formation can be understood in terms of the standard Big Bang model of
cosmology. In this model, the universe is composed predominantly of dark
matter and dark energy, and began with a Big Bang about 13.7 billion
years ago.

The team of scientists found El Gordo using ACT
thanks to the Sunyaev-Zeldovich effect. In this phenomenon, photons in
the cosmic microwave background interact with electrons in the hot gas
that pervades these enormous galaxy clusters. The photons acquire energy
from this interaction, which distorts the signal from the microwave
background in the direction of the clusters. The magnitude of this
distortion depends on the density and temperature of the hot electrons
and the physical size of the cluster.

X-ray data from Chandra
and the European Southern Observatory's Very Large Telescope, an 8-meter
optical observatory in Chile, show El Gordo is, in fact, the site of
two galaxy clusters colliding at several million miles per hour. This
and other characteristics make El Gordo akin to the well-known object
called the Bullet Cluster, which is located almost 4 billion light years
closer to Earth.

X-ray image of El Gordo

As with the Bullet Cluster, there is evidence
that normal matter, mainly composed of hot, X-ray bright gas, has been
wrenched apart from the dark matter in El Gordo. The hot gas in each
cluster was slowed down by the collision, but the dark matter was not.

"This is the first time we've found a system like the Bullet Cluster at
such a large distance," said Cristobal Sifon of Pontificia Universidad
de Catolica de Chile (PUC) in Santiago. "It's like the expression says:
if you want to understand where you're going, you have to know where
you've been."

These results on El Gordo are being announced at
the 219th meeting of the American Astronomical Society in Austin, Texas.
A paper describing these results has been accepted for publication in
The Astrophysical Journal.

NASA's Marshall Space Flight Center
in Huntsville, Ala., manages the Chandra program for NASA's Science
Mission Directorate in Washington. The Smithsonian Astrophysical
Observatory controls Chandra's science and flight operations from
Cambridge, Mass.