Puzzling galaxy cluster not so puzzling after all, maybe

Astronomers have been taking another look at a merging galaxy cluster that threw up some surprising results earlier this year, and they think that it’s behavior is not so strange, after all.

Many astronomers have been taking a close look at merging galaxy clusters, because they may be able to help resolve one of the big mysteries of cosmology — the nature of dark matter.

A galaxy cluster is a vast object containing hundreds of conventional visible galaxies, dark matter and hot gas. When two  clusters run into each other, the visible galaxies past straight through because the distances between them are so huge. According to our current theories, the dark matter should follow with the galaxies, because dark matter does not interact with other matter except through gravity. The gas should collide and get stuck in the middle.

And that is the pattern that astronomers have mostly seen in merging clusters, such as this study of the “Musket Ball” or “Perry’s Cluster” led by UC Davis graduate student Will Dawson.

But earlier this year UC Davis astronomer James Jee and colleagues published observations of merging cluster Abell 520, using the Hubble Space Telescope, showing a “dark core” of dark matter, with few galaxies, left in the middle of the cluster while most of the visible galaxies had moved away.

Now another team, led by Douglas Clowe at Ohio State University and including another UC Davis astronomer, assistant professor of physics Marusa Bradac, has taken another look at Abell 520 and come up with a different result.

Because dark matter is invisible but exerts gravity, its location can only be observed by measuring how it bends and distorts light from visible galaxies in the background from our point of view. This technique is called “gravitational lensing.”

Clowe, Bradac and colleagues used the Advanced Camera for Surveys on the Hubble Space Telescope to make dark matter in the cluster, while Jee’s team had used the Hubble’s Wide Field Planetary Camera as well as Earth-based telescopes. The new measurements found an amount and distribution of dark matter that was closer to that predicted by current theory.

In an email, Jee said that his group had also been looking at Clowe’s data.

“However, we find that the ACS results are consistent with our previous WFPC2 results. Although we believe Clowe et al. (2012) did a careful analysis, it seems that their correction for the systematics might have been imperfect and the cause of the discrepancy,” he said.

Jee is writing a new paper on Abell 520 that he hopes will be accepted soon. “We hope some great science is learned from this debate,” he wrote.

More: UC Davis, UC Irvine astronomers form group to study galaxy clusters

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