The Big Crash: Satellites Reveal a New “Super-Galactic” Collision and How it May Effect Our Understanding of the Universe
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July 19, 2007

The Big Crash: Satellites Reveal a New “Super-Galactic” Collision and How it May Effect Our Understanding of the Universe

Galaxies We often think of galaxies as self-contained units, but in actuality galaxies interact and merge relatively frequently. In fact, our own galaxy is swallowing up another as we speak. It is difficult to conceive of the laws of nature in such a broad scope. Imagine not just one galaxy, but a whole cluster of galaxies crashing into each other—all whilst we are blissfully unaware of how this effects us on this one tiny planet twirling about in this huge cosmic dance.

It is believed that collisions between galaxies are some of the most important elements in driving cosmic evolution over time. Most galaxies have likely had interactions with other galaxies from the time they first formed.

While smaller collisions are more common, major galaxy cluster-to-cluster collisions are thought to be quite rare. The estimated frequency ranges from less than one in a thousand clusters to one in a hundred. However, two orbiting telescopes Chandra and the XXM-Newton recently caught a pair of galaxy clusters merging into one giant. By studying this new discovery, researchers were able to confirm that galaxy clusters can collide much more quickly than has been previously been deemed possible.

When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem that ensues.

The Bullet Cluster is a previously discovered, much-studied pair of galaxy clusters, which have already collided head on. One cluster has now passed through the other, like a bullet traveling through an apple. In the Bullet Cluster, this is happening across our line of sight, so we can see the event clearly.

This newly discovered cluster, Abell 576, is seen head on, so one cluster is now almost directly behind the other. The ‘cold’ clouds of gas are the cores of each cluster, which have survived the initial collision but will eventually fall back together to become one giant. Renato Dupke and colleagues from the University of Michigan used the ESA’s XMM-Newton and NASA’s Chandra orbiting observatories, to disentangle the puzzle.

The data reveals that the clusters have collided at a speed of over 3300 km/s. This is particularly interesting because computer models of colliding galaxy clusters has suggested that such a high speed would be impossible to reach.

Nevertheless, the Bullet Cluster is estimated to have a collision speed similar to the Abell 576 system. “There is now a growing body of evidence that these high collision velocities are possible,” says Dupke. The job of explaining how these high-speed collisions occur now rests with the astrophysicists.

When major cluster-cluster collisions occur, their internal gas is thrown out of equilibrium and if unrecognized, causes underestimation of its mass by between 5 and 20 percent.

This is extremely important for scientists to better understand, because the masses of the various galaxy clusters are used to estimate the cosmological parameters that describe how the Universe expands. Therefore, identifying colliding systems is quite important to our understanding of the Universe.

Since a galaxy is made of roughly 100 billion stars, one might think that in a head on collision between two galaxies, there would be incredible collisions between those stars, but the actual probability is almost 0. Why? Because the stars are spaced so far apart, the stars of one galaxy pass through the stars of another galaxy without much chance of direct impact. However, they do interact gravitationally. The galaxies “pull” on the material in the other galaxies and disrupt their morphologies. There is also friction between the other materials and gas in the colliding galaxies, which can cause shock waves that can trigger new star formation.

Dupke and colleagues are already investigating a number of other clusters that also appear to be interacting. Once we understand the interactions between these mega-clusters, we will better understand the nature of our universe.

Posted by Rebecca Sato

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