Washington — An exceptional galaxy cluster, the largest seen in the
distant universe, has been found 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 of Rutgers University in New Brunswick, N.J., who led the
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, also of 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.
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
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.
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