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A Cosmic Crash of Unexpected Proportions

An international research team investigates the distant galaxy cluster Abell 2744

The biggest known cosmic collision in the Universe took place in a distant galaxy cluster called Abell 2744. That is the conclusion of an international team of scientists investigating the debris of this massive crash with novel research methods that were developed at Heidelberg University’s Institute of Theoretical Astrophysics.

The image shows the galaxy cluster Abell 2744. It combines observations in the visible light spectrum with X-ray images of satellite Chandra (red) and clouds of dark matter (blue). The particularities of the system are clearly visible, for example a clump of dark matter with no stars or gas (northwest sector) and a clump of galaxies and dark matter without gas (western sector). The scale indicates a distance of 250,000 parsecs, approximately nine times the diameter of the visible part of our own galaxy, the Milky Way. In astronomy, the compass points East and West are switched, as shown at the bottom right corner of the image.
Image source: NASA, ESA, ESO, CXC, J. Merten (Heidelberg/Bologna) & D. Coe (STScl)

These methods enabled the scientists to reconstruct the course of events over a period of several hundreds of millions of years and thus to understand how large-scale structures develop in the Universe based on the interaction of different kinds of matter. Researchers from Brazil, Canada, Germany, Israel, Italy, Scotland, Spain, Taiwan and the United States of America collaborated on the investigation.

The astrophysicists observed the galaxy cluster Abell 2744 from an unprecedented number of angles with high-performance telescopes, among them the Very Large Telescope of the European Southern Observatory (ESO) in Chile, the Japanese Subaru Telescope in Hawaii and the Hubble and Chandra space telescopes. With the data gleaned from these observations, the research team headed by astrophysicist Dr. Julian Merten of the Heidelberg Institute of Theoretical Astrophysics was able to investigate the three essential components of galaxy clusters: galaxies and their stars, intergalactic gas and dark matter.

Each of the approx. 1,000 galaxies of Abell 2744 contains many billions of stars. However, this “visible” matter only makes up about five percent of the entire mass of the galaxy cluster. The galaxies “float” in the diffuse gas that is distributed between them, Dr. Merten explains. This “intergalactic gas” comprises 20 percent of the overall mass and was heated up so intensely by the effects of gravitational forces in the galaxy cluster that it emits radiation mostly in the X-ray wavelength band. The remaining 75 percent of the galaxy cluster consist of the mysterious “dark matter”.

To understand the processes going on in Abell 2744, the scientists aimed to determine the distribution of these three components as precisely as possible. This is easily accomplished for galaxies and intergalactic gas, but dark matter is much harder to pin down. It neither emits nor absorbs light and can only be detected through its gravitational attraction. However, during his time as PhD student at the Heidelberg Graduate School of Fundamental Physics, Julian Merten devised special methods for measuring the distribution of dark matter with the aid of an effect known as gravitational lensing.

When light rays emitted by galaxies far beyond Abell 2744 cut through the massive galaxy cluster, the gravitational attraction of the unevenly distributed dark matter changes the trajectory of the light travelling through the cluster. “The rays of light are ‘bent’ more or less strongly so that the images of the background galaxies appear distorted in a characteristic way,” says Dr. Merten. “By analysing this distortion for a large number of background galaxies we are able to chart out a map showing the distribution of dark matter.”

The surprising outcome of the analysis of Abell 2744 is that this system consists of at least four different galaxy clusters that must have collided over a period of about 350 million years. “The collision obviously separated the hot gas from the dark matter and led to an unusual and fascinating distribution of the three kinds of matter,” adds Dr. Merten. In the northwest sector, the scientists found an area where dark matter was separated from the other components in an unusual way. The hot gas leads the dark matter by a large distance and the galaxies do not appear to match the position of the dark matter, either. In the western sector, the researchers came across an area that contains both dark matter and galaxies, but no hot gas. “It looks as if this gas was stripped away completely in the central region of the cluster during the collision, and was left behind,” says Dr. Merten. Because of the large number of unusual and often mysterious phenomena, the researchers have dubbed Abell 2744 “Pandora’s cluster”.

A publication on these research findings entitled “Creation of Cosmic Structure in the Complex Galaxy Cluster Merger Abell 2744” will be appearing in ”Monthly Notices of the Royal Astronomical Society”. Preprint:

The Institute of Theoretical Astrophysics is part of Heidelberg University's Centre for Astronomy (ZAH).

Dr. Julian Merten
Zentrum für Astronomie der Universität Heidelberg (ZAH)
Institute of Theoretical Astrophysics
phone: +49 6221 54 8987
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Marietta Fuhrmann-Koch | idw
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