Cosmic Heavyweights in Free-For-All

The most crowded collision of galaxy clusters has been identified by
combining information from three different telescopes. This result
gives scientists a chance to learn what happens when some of the
largest objects in the Universe go at each other in a cosmic
free-for-all.

Using data from NASA’s Chandra X-ray Observatory, Hubble Space
Telescope and the Keck Observatory on Mauna Kea, Hawaii, astronomers
were able to determine the three-dimensional geometry and motion in
the system MACSJ0717.5+3745 (or MACSJ0717 for short) located about 5.4
billion light years from Earth.

The researchers found that four separate galaxy clusters are involved
in a triple merger, the first time such a phenomenon has been
documented. Galaxy clusters are the largest objects bound by gravity
in the Universe.

In MACSJ0717, a 13-million-light-year-long stream of galaxies, gas and
dark matter — known as a filament — is pouring into a region already
full of galaxies. Like a freeway of cars emptying into a full parking
lot, this flow of galaxies has caused one collision after another.

This composite image shows the massive galaxy cluster MACSJ0717.5+3745 (MACSJ0717, for short), where four separate galaxy clusters have been involved in a collision, the first time such a phenomenon has been documented. Hot gas is shown in an image from NASA’s Chandra X-ray Observatory and galaxies are shown in an optical image from NASA’s Hubble Space Telescope. The hot gas is color-coded to show temperature, similar to a temperature map of the Earth given in a weather forecast. In MACSJ0717 the coolest gas is shown as reddish purple, the hottest gas is blue and the temperatures in between are purple.

“In addition to this enormous pileup, MACSJ0717 is also remarkable
because of its temperature,” said Cheng-Jiun Ma of the University of
Hawaii and lead author of the study. “Since each of these collisions
releases energy in the form of heat, MACS0717 has one of the highest
temperatures ever seen in such a system.”

While the filament leading into MACJ0717 had been previously
discovered, these results show for the first time that it was the
source of this galactic pummeling. 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. Secondly, the largest
hot region in MACSJ0717 is 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,” said team
member Harald Ebeling, also from University of Hawaii. “This is a
wonderful system for studying how clusters grow as material falls into
them along 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,” said Ma.

Multiwavelength data were crucial for this work. The optical data from
Hubble and Keck give information about the motion and density of
galaxies along the line of sight, but not about their course
perpendicular to that direction. By combining the X-ray and optical
data, scientists were able to determine the three-dimensional geometry
and motion in the system.

In the future, Ma and his team hope to use even deeper X-ray data to
measure the temperature of gas over the full 13-million-light-year
extent of the filament. Much remains to be learned about the
properties of hot gas in filaments and whether its infall along these
structures can significantly heat the gas in clusters over large
scales.

“This is the most spectacular and most disturbed cluster I have ever
seen,” says Ma, “and we think that we can learn a whole lot more from
it about how structure in our Universe grows and evolves.”

The paper describing these results appeared in the March 10th issue of
the Astrophysical Journal Letters. 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.

More information, including images and other multimedia, can be found at:
http://chandra.harvard.edu
and
http://chandra.nasa.gov