When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Recognising the signs of collisions between whole clusters of galaxies, however, is not as easy.
The Bullet Cluster
Dupke realised that Abell 576 is also a collision, but 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.The data reveals that the clusters have collided at a speed of over 3300 km/s. This is interesting because there are some computer models of colliding galaxy clusters that suggest that such a high speed is impossible to reach.
Major cluster-cluster collisions are expected to be rare, with estimates of their frequency ranging from less than one in a thousand clusters to one in a hundred. On collision, their internal gas is thrown out of equilibrium and if unrecognised, causes underestimation of its mass by between 5 and 20 percent.
This is important because the masses of the various galaxy clusters are used to estimate the cosmological parameters that describe how the Universe expands. So, identifying colliding systems is extremely important to our understanding of the Universe.
Dupke and colleagues are already investigating a number of other clusters that also appear to be interacting.
Norbert Schartel | EurekAlert!
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