The Awesome Power of Galaxy Cluster Mergers

The scales are mind-boggling and the physics is cutting edge, so how do you go about simulating the collision of two galactic clusters? Using some of the most powerful computers in the world, researchers at Argonne National Laboratory, the Flash Center at the University of Chicago and the Harvard-Smithsonian Center for Astrophysics have done just that.

A galactic cluster is a group of galaxies held together under their mutual gravity. Occasionally -- during universal history spanning time scales of billions of years -- two clusters will slam into each other at breakneck speeds. But considering these collisions occur in volumes of space measuring in the megaparsecs (one megaparsec is equal to over 3.2 million light-years), it's easy to see why these events take billions of years to merge.

The study of galactic cluster collisions is very important for cosmologists to understand, amongst other things, dark matter. Dark matter, the cosmic phantom, is thought to make up the majority of mass in our Universe. But the problem with dark matter is that it doesn't interact with normal matter, so it cannot be observed directly.

The presence of dark matter can only be observed indirectly, so this means you have to see how the stuff interferes with normal matter gravitationally. To do this, astronomers have to look at the biggest structures in our Universe to stand a hope of detecting it. Also, by simulating cluster collisions and mergers, we can better understand what astronomers are seeing.

In 2006, astronomers announced the first detection of dark matter inside the unfolding collision of two galactic clusters in what is collectively known as the Bullet Cluster. The distribution of normal matter (i.e. the visible gas and galaxies making up the clusters) suggested there had to be another component in the mix: dark matter.

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Now, scientists have simulated this awesome collision using a code that creates a 3D representation of two clusters colliding. Looking at how the animation (below) develops, it may as well be the life story of the Bullet Cluster itself.

The code simulates the interaction of both normal matter and dark matter. The normal matter interacts, mixes and generates turbulence as the collision progresses. For this component of the simulation they use a hydrodynamic code -- it's basically the science of how two liquids or gases mix. As the two clouds of dark matter inside each cluster can only interact gravitationally (the dark matter particles cannot collide, or scatter, via any other mechanism), each particle is modeled individually. This an N-body code.

Once the simulation is sent on its way, the mixing of normal matter is simulated along with the dark matter. As the dark matter cores from both clusters reach an equilibrium state, orbiting inside the post-collision cluster, its motion gravitationally mixes the normal matter.

"The dark matter cores slip past and through each other,whereas the two gas components interact and mix," narrator Carrie Eder explains during the collision visualization.

"It is also clearly seen that the mixing of the gas is driven completely by the violent orbital motion of the dark matter cores."