Colossal Galaxy Cluster from Early Universe –Could Reveal Secrets of Dark Matter

 

          
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The El Gordo galaxy cluster— which means "the fat one" in Spanish — is officially known as ACT-CL J0102-4915 located located more than 7 billion light-years from Earth. Astronomers hope it could one day reveal secrets about the invisible dark matter that fills the universe. The monster galaxy cluster has mass about 2 quadrillion times that of the sun, making it the most massive known cluster in the observable universe.


Galaxies within the cluster are concentrated in two distinct groups, and gas in El Gordo can reach super-high temperatures of nearly 360 million degrees Fahrenheit (200 million degrees Celsius), based on X-rays collected by Chandra and the European Southern Observatory's Very Large Telescope in Chile which suggests that cluster is the site of a violent merger between two galaxy clusters.

El Gordo was discovered using NASA's Chandra X-ray Observatory in space and the Atacama Cosmology Telescope in Chile. The scientists detailed their findings at the annual meeting of the American Astronomical Society during a presentation that included a separate announcement of the discovery of the most distant galaxy cluster ever seen in the early universe.

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 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 that El Gordo is, in fact, the site of two galaxy clusters running into one another 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."

The Daily Galaxy via http://chandra.si.edu

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