Ongoing research by an international team of astronomers is providing new insights into cataclysmic cosmic collisions between galaxy clusters.

Using the world’s most powerful X-ray space observatory, the team is unraveling the complex interactions that take place in the “traffic pile-ups” that occur as clusters containing hundreds of galaxies and trillions of solar masses of gas and dark matter interact and merge.

Speaking on Friday 8 April at the RAS National Astronomy Meeting in Birmingham, Dr. Elena Belsole (University of Bristol) will present new results obtained with ESA’s orbiting XMM-Newton observatory. The images and other data reveal an environment racked by violent shock waves that squeeze and compress the intra-cluster gas, raising its temperature to many millions of degrees.

Galaxy clusters, measuring up to 6 million light years across, are the largest objects whose mass can be measured by astronomers. From observations of many clusters, it is possible to estimate the distribution of mass in the Universe as a whole. This provides important information about what the Universe is made of, how it began, and how will it end.

However, only 5% of the mass of galaxy clusters lies in stars and galaxies. The space between the galaxies is filled with gas which is so hot (10 – 100 million degrees Celsius) that it can only be seen at X-ray wavelengths.

How did the gas between the galaxies get so hot? Galaxy clusters grow through the action of gravity, continuously pulling in smaller galaxy systems and undergoing the occasional violent collision with an object of comparable size.

In such events, the clusters start to feel each other’s gravitational pull: they interact and, after a prolonged period, they finally merge. These mergers are the most energetic events to have taken place in the Universe since the Big Bang. The energy released in cluster collisions irreversibly modifies the physical conditions within a cluster through compression waves and shocks which heat the gas to temperatures 10,000 times those on the surface of the Sun.

By using space-based instruments able to see at X-ray wavelengths, Belsole’s team has been able to measure the origins and energy of X-rays from galaxy clusters. From the positional information, they were able to map the distribution of the gas in the clusters. From the X-ray energy, they were able to measure the gas temperature. By combining the two, they could map the temperature structure of the cluster gas.

The temperature is the key quantity which allows the scientists to discriminate between clusters which are undergoing dramatic collisions and those which are not. The temperature shows directly the conversion of enormous amounts of kinetic energy into the thermal energy which heats the gas.

“Thanks to observations obtained with XMM-Newton, the most powerful X-ray detector ever built, we are now able to describe fully the gas in galaxy clusters,” said Belsole.

“From the temperature, we calculate that clusters can collide at velocities greater than 2,000 km/s. We observe that clusters are unique in their morphology and temperature distribution, and it is through these differences that we can say whether a cluster is young or old.”

Belsole’s team has recently investigated three different merging clusters each made up of hundreds of galaxies. One of these, known as Abell 1750 (A1750), is a young merger located 1.1 billion light years from Earth. It involves two clusters, separated by more than 3 million light years, which are just starting to interact.

Each of these colliding clusters has a total mass about 500 trillion times that of the Sun and is moving at a speed of around 1,400 km/s. The violent interaction between them causes shocks and compression of the intra-cluster gas, producing an arc-like region of gas between the two with a temperature of 70 million degrees Celsius. The collision will reach its climax in 1 – 2 billion years, when the cores collide and the energy release is at its maximum.

A more complicated example is A3266, located 800 million light years from Earth. Two clusters, of unequal mass, are seen just after the point of closest encounter. This creates a hot, boomerang-shaped region where a shock wave is propagating in the direction of motion of the smaller, infalling cluster. A1750 will look like this in 1 – 2 billion years.

An older example is A3921, located 1.2 billion light years from Earth. In this case, the very asymmetric morphology and temperature distribution reveal that two clusters, again of unequal mass, have already had their first encounter. The smaller cluster, about three times less massive than the main cluster, was almost totally destroyed by the encounter. The collision shredded the smaller cluster, at the same time producing a hot region of shocked gas stretching from the centre of the main cluster.

“This research shows the violent manner by which the largest structures in the Universe form, and that the formation has happened in the recent past,” said Belsole. “The process is still taking place today. In several billion years, the group of which our galaxy, the Milky Way, is a member, will be torn apart as it merges with the nearby Virgo cluster.”

The ongoing research is described in several papers (see below). Studies of A3266 will be published in a forthcoming issue of Astronomy and Astrophysics.


During the NAM (5 – 8 April), Dr. Belsole can be contacted via the NAM press office (see above).

Normal contact details:

Dr. Elena Belsole
University of Bristol
Tel: +44 (0)117-9288698


These studies have been undertaken in collaboration with:


  • . Jean-Luc Sauvageot – CEA-Saclay, France
  • Dr. Gabriel W. Pratt – Max Planck Institute fuer extraterrestriche Physik,
  • Garching, Germany
  • Dr. Herve Bourdin – Universita “Tor Vergata”, Rome, Italy

Previous papers:

Title: An XMM-Newton observation of the dynamically active binary cluster A1750.
Authors: E. Belsole, G.W. Pratt, J.-L. Sauvageot, H. Bourdin, 2004.
Journal: Astronomy and Astrophysics, vol. 415, page 821.

Title: An XMM-Newton observation of A3921: an off-axis merger.
Authors: E. Belsole, J.-L. Sauvageot, G.W. Pratt, H. Bourdin, 2005,
Journal: Astronomy and Astrophysics, vol. 430, pages 385-397.

The 2005 RAS National Astronomy Meeting is hosted by the University of Birmingham, and sponsored by the Royal Astronomical and the UK Particle Physics and Astronomy Research Council (PPARC).


X-ray images of merging galaxy clusters and their optical counterparts can be found at:

Image Captions

A1750: This image shows two views of the two main components of the multiple galaxy cluster Abell 1750: A1750 N to the North, and A1750 C to the South. On the left, the contours of the X-ray emission are overlaid on a Digital Sky Survey optical image of the cluster. The two X-ray peaks are clearly centred on the giant elliptical galaxies at the centre of each cluster, which are separated by a projected distance of about 3 million light years. The true distance between the two peaks is larger than this, since the clusters are seen projected onto the plane of the sky. On the right, the same X-ray contours are overlaid on a temperature map of the cluster. It can be seen that A1750 N is cooler than A1750 C, meaning that it is slightly less massive. An arc-like hot region can be seen between the clusters, which indicates that the gas is being shocked and compressed as the two clusters come together at a speed of around 1400 km/s. The hot regions associated with A1750 C are regions of shocked gas from an older merger which occurred about 1 – 2 billion years ago.

A3266: This image shows two views of the galaxy cluster Abell 3266. On the left, the contours of the X-ray emission are overlaid on a Digital Sky Survey optical image of the cluster. The cluster seems fairly symmetric. However the temperature map, on the right, shows an extraordinary, boomerang-shaped hot region wrapped around the cluster core. It appears that a small subcluster is ploughing through the centre of the main cluster at more than 2000 km/s, having come from the top left of the image, creating the giant shock that we see here. In 1-2 billion years, A1750 will look like this.

A3921: This image shows two views of the galaxy cluster Abell 3921. On the left, the contours of the X-ray emission are overlaid on a Digital Sky Survey optical image of the cluster. The main X-ray peak is clearly centred on the giant elliptical galaxy at the centre of the main cluster. The secondary X-ray peak, to the upper right, is the remains of a smaller cluster which passed through the body of main cluster about 500 million years ago. Its passage created the hot shocked regions visible in the temperature map on the right, with gas heated up to 90 million degrees Celsius.