Cosmic crash. El Gordo consists of two clusters in collision, as revealed by the two separate swarms of individual galaxies (red) and the asymmetric cloud of hot, x-ray emitting gas (blue) in between.


When Galaxies Crash

AUSTIN, TEXAS—It's hot, it's massive, it's radiant, and it's very far away. Nicknamed El Gordo ("the fat one"), ACT-CL J0102-4915 is the biggest cluster of galaxies ever found in the distant universe. With good reason: it appears to be two clusters in collision.

A team of astronomers led by Felipe Menanteau and John Hughes of Rutgers University in Piscataway, New Jersey, discovered El Gordo with the Atacama Cosmology Telescope (ACT) in Chile. It betrayed its existence by a shadowy imprint in the cosmic background radiation—the "afterglow" of the Big Bang.

Follow-up studies with large telescopes on the ground revealed that the cluster is 7 billion light-years away, which means its light took 7 billion years to reach us. So we are seeing the cluster as it appeared 7 billion years ago, when the universe was only half its present age.

Observations with NASA's Chandra X-ray Observatory showed that El Gordo contains x-ray emitting gas at a temperature of 200 million°C. And using the European Very Large Telescope in Chile, the team measured the velocities of the individual galaxies in the cluster, averaging some 5 million kilometers per hour.

From all these data, the scientists determined the cluster's mass: two quadrillion times the mass of our own sun, most of it dark matter and hot, tenuous gas. By studying the distribution of the x-ray emitting gas and the individual galaxies in the cluster, the team also concluded that El Gordo is actually two clusters in collision. The results were presented here today at the 219th meeting of the American Astronomical Society.

Finding two galaxy clusters in collision is nothing new. In fact, at the same meeting another team of astronomers led by William Dawson of the University of California, Davis, announced the discovery of the Musket Ball Cluster, somewhat closer to Earth, which also consists of two smaller clusters in the process of merging. But El Gordo is the most massive and the hottest cluster found so far at this relatively early stage in cosmic history.

Galaxy clusters need time to grow from small, denser regions in the primordial gas that filled the newborn universe. So the team wondered whether current theories about the evolution of the universe allow for the growth of a massive structure like El Gordo in less than 7 billion years. If not, something must be wrong with the theory. But Hughes says computer simulations of the formation of structure in the universe reveal that it's just barely possible. "Heavyweight clusters like El Gordo must be very rare, but they're not in conflict with our ideas about structure formation in the Universe," he says.

The Musket Ball Cluster found by Dawson and his colleagues appears to be in a more advanced stage of merging than El Gordo does. The Musket Ball Cluster's dark matter, the hot gas, and the individual galaxies are strongly separated from each other because they behave differently during a cluster collision. Dawson says studying cluster collisions and mergers at various stages is bound to shed more light on the process that builds the largest structures in the universe.