Cluster Heavyweights Caught in Cosmic Traffic Jam

Astronomers have recently identified the Universe’s most crowded cosmic free-way, where monster galaxy clusters are slamming together in one of the largest collisions ever recorded. Pinpointing such a pile-up required data from three of the world’s best telescopes, and the discovery now provides scientists with a chance to watch what happens when some of the Universe’s largest objects collide.

A team of astronomers from the University of Hawai’i used the W. M. Keck Observatory on Mauna Kea, Hawai’i, and the space-based Chandra X-ray Observatory and Hubble Space Telescope, both NASA missions, to collect their data. The team then calculated the three-dimensional geometry and motion in the system MACSJ0717.5+3745, or MACSJ0717 for short, which is located about 5.4 billion light years from Earth in the direction of the Hydra constellation.

The researchers’ new data shows that four separate galaxy clusters are involved in a triple collision. Galaxy clusters are the largest objects bound by gravity in the Universe, and it is the first time astronomers have ever observed such a massive merger. Only by observing such violent collisions can astronomers better understand how clusters grow, says the team’s lead researcher Cheng-Jiun Ma of the University of Hawai’i.

To make such observations, the astronomers used the Keck II telescope and its DEIMOS instrument from 2004 to 2008, as well as the Hubble Space Telescope in 2004, as part of an ongoing research project to learn more about cluster and galaxy evolution.

Taking spectroscopic data is the only way to measure galaxy velocities along the line of sight. This data adds the crucial third dimension to what the team can learn about the dynamics of a large cluster from its gas and galaxy densities, says team member Harald Ebeling, also of the University of Hawai’i. The DEIMOS instrument’s large field of view and Keck’s massive light gathering power make the instrument and telescope the only research tool that can survey a structure as large as MACSJ0717 in a reasonable amount of time.

“After all, this cluster is at a few billions of light years from Earth, meaning it takes one to two hours even with Keck to measure reliable radial velocities for cluster galaxies,” Ebeling notes.

The optical data provides information about the radial motion and density of galaxies. The team then combined the Hubble and Keck data with information taken with the Chandra X-ray Observatory to determine the three-dimensional geometry and motion in the system and how that motion affects the collisions.

The data revealed a large, hot region located where a long stream of galaxies and gas—known as a filament—runs into MACSJ0717. The filament extends 13 million light years and pours gas and galaxies into the system like cars exiting a crowded interstate into a full parking lot. This influx of matter causes one collision after another.

Each of the collisions releases energy in the form of heat. MACSJ0717 therefore has one of the highest temperatures ever seen in such a system, Ma explains.

The filament leading into MACSJ0717 had been previously discovered. But the new results show that it is the source of the cluster collisions. The evidence is two-fold. First, by comparing the position of the gas and clusters of galaxies, the researchers tracked the direction of clusters’ motions, which matched the orientation of the filament in most cases. Second, the large hot region in MACSJ0717 is located where the filament intersects the cluster, suggesting ongoing impacts.

“MACSJ0717 shows how giant galaxy clusters interact with their environment on scales of many millions of light years,” Ebeling says. “This is a wonderful system for studying how clusters grow as material falls into them along the filaments.”

Computer simulations show that the most massive galaxy clusters should grow in regions where large-scale filaments of intergalactic gas, galaxies, and dark matter intersect, and material falls inward along the filaments.

“It’s exciting that the data we get from MACSJ0717 appear to beautifully match the scenario depicted in the simulations,” Ma adds.

In the future, he and his colleagues want to use even deeper X-ray data to measure the temperature of gas over the full 13 million light year extent of the filament to learn more about how structure in the Universe grows and evolves.

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. NASA Goddard Space Flight Center in Greenbelt, MD performs the daily orbital operations, servicing mission development, and overall management of the Hubble Program. The Space Telescope Science Institute (STScI) in Baltimore, MD develops and executes Hubble’s scientific program and is managed by the Association of Universities for Research in Astronomy (AURA) under contract to NASA.

The W. M. Keck Observatory operates twin 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i. The two telescopes feature a suite of advanced instrumentation including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectrographs and a world-leading laser-guide-star adaptive optics system. The Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. For information please call 808.885.7887 or visit