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January 13, 2013

The Train Wreck Cluster, a galaxy cluster in Orion

Abell 520

Image Credit: NASA, ESA, CFHT, and M.J. Jee (University of California, Davis)

The Train Wreck Cluster (Abell 520) is a galaxy cluster of 5.4 million light-years across that contains about 300 galaxies. It lies some 2.65 billion light-years away in the constellation of Orion, while moving away from us at approximately 1,066 kilometers per second. The cluster has gotten its nickname because of its unusual chaotic structure due to the high-speed collision between smaller galaxy clusters.

When galaxies collide, three ingredients come into play: 1) individual galaxies and their billions of stars, 2) hot gas in between the galaxies, and 3) the mysterious dark matter which is thought to be the gravitational “glue” that holds galaxies together, and that actually makes up the bulk of the mass. Dark matter can only be detected through its gravitational effects.

During gigantic collisions like this, astronomers believed that the dark matter and the galaxies should stick together, even during the most violent collisions, as was seen in another galaxy collision (the Bullet Cluster). But in the collision of Abell 520, something surprising was seen.

A dark matter core was found, containing hot gas, but no galaxies, which normally would be seen in the same location as the dark matter. For some reason, the galaxies were stripped away from the densest part of the dark matter. In addition to this core, a corresponding “light region” was found, which had galaxies, but little or no dark matter. Somehow this collision separated the dark matter from the regular matter.

We know that dark matter is affected by gravity, but it is supposed to barely interact with other matter or with itself: during an encounter, clouds of dark matter flow right through each other without slowing down. So what could have stripped these two apart?

Several explanations were suggested, but all these explanations would have posed problems for current theories.
Recently however, a new observation of Abell 520 found that the core does not appear to be over-dense in dark matter after all. In this new study astronomers estimated the amount of dark matter in the cluster by measuring the amount of gravitational “shear” in Hubble images.

Shear is the warping and stretching of galaxies by the gravity of dark matter. More warping indicates the presence of more gravity than is inferred from the presence of luminous matter, therefore requiring the presence of dark matter to explain the observation. Former observations could have introduced anomalous shear and not a measure of the dark matter distribution.

Now, less shear was measured in the cluster’s core than was previously found. In this new study the ratio of dark matter to normal matter, in the form of stars and gas, is 2.5 to 1, which is what astronomers expected. The earlier observation, however, showed a 6-to-1 ratio of dark matter to normal matter, which challenged theories of how dark matter behaves.

This natural-color image of the galaxy cluster was taken with NASA’s Hubble Space Telescope and the Canada-France-Hawaii Telescope in Hawaii.

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