Galaxy clusters are among the most massive structures in the Universe. Every galaxy cluster contains a galaxy that is brighter than the other members, aptly named the brightest cluster galaxy (BCG). Astronomers using the NASA/ESA Hubble Space Telescope have discovered that BCGs ‘wobble’ relative to the cluster’s center of mass. According to the team, this result is inconsistent with predictions made by the current standard model of dark matter.
This Hubble image shows the massive galaxy cluster Abell S1063. Image credit: NASA / ESA / J. Lotz, STScI.
Dark matter constitutes just over 25% of all matter in the Universe but cannot be directly observed, making it one of the biggest mysteries in modern astronomy. Invisible halos of elusive dark matter enclose galaxies and galaxy clusters alike.
The cold dark matter model predicts that once a galaxy cluster has returned to a ‘relaxed’ state after experiencing the turbulence of a merging event, the BCG does not move from the cluster’s center. It is held in place by the enormous gravitational influence of dark matter.
But now, EPFL astronomer David Harvey and co-authors have analyzed 10 massive galaxy clusters observed with Hubble, and found that their BCGs are not fixed at the center as expected.
The Hubble data indicate that they are ‘wobbling’ around the center of mass of each cluster long after the galaxy cluster has returned to a relaxed state following a merger.
In other words, the center of the visible parts of each galaxy cluster and the center of the total mass of the cluster — including its dark matter halo — are offset, by as much as 40,000 light-years.
If this ‘wobbling’ is not an unknown astrophysical phenomenon and in fact the result of the behavior of dark matter, then it is inconsistent with the standard model of dark matter and can only be explained if dark matter particles can interact with each other — a strong contradiction to the current understanding of dark matter.
This may indicate that new fundamental physics is required to solve the mystery of dark matter.
“We found that that BCGs ‘slosh’ around at the bottom of the halos,” Dr. Harvey said.
“This indicates that, instead of a dense region in the center of the galaxy cluster, there is a much shallower central density — a striking signal of exotic forms of dark matter right at the heart of galaxy clusters.”
“We’re looking forward to larger surveys — such as the Euclid survey — that will extend our dataset,” added co-author Dr. Frederic Courbin, also of EPFL.
“Then we can determine whether the wobbling of BGCs is the result of a novel astrophysical phenomenon or new fundamental physics. Both of which would be exciting.”
The results are published in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
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David Harvey et al. A detection of wobbling Brightest Cluster Galaxies within massive galaxy clusters. MNRAS, published online October 26, 2017; doi: 10.1093/mnras/stx2084
