Galaxy clusters are the
largest known structures in the Universe, containing thousands of galaxies and
hot gas. But more importantly, they contain the mysterious dark matter, which
accounts for 27 percent of all matter and energy. Current models of dark matter
predict that galaxy clusters have very dense cores, and those cores contain a
very massive galaxy that never moves from the cluster's center.
But after studying ten
galaxy clusters, David Harvey at EPFL's Laboratory of Astrophysics and his
colleagues in France and the UK have discovered that the density is much
smaller than predicted, and that the galaxy at the center actually moves.
Every galaxy cluster
contains a galaxy that is brighter than the others, aptly named "brightest
cluster galaxy" or BCG. Recent evidence from simulations of exotic,
non-standard dark matter shows that BCGs actually wobble long after the galaxy
cluster has relaxed. This is residual wobbling caused by massive merging of
galaxy clusters.
The researchers compared
their observations to the predictions from the BAHAMAS suite of cosmological
hydro-dynamical simulations finding that the two did not match. According to
the Standard Model of dark matter (called "cold dark matter"), this
wobbling doesn't exist because the enormous density of dark matter keeps it
tightly bound at the center of the galaxy cluster.
Therefore, this mismatch
suggests the existence of yet-unknown physics that have not been accounted for.
The galaxy clusters that the
astronomers studied also act as strong gravitational lenses: they are so
massive that they warp spacetime enough to distort light passing through them,
like a lens. As a result, they can be used to make a map of dark matter,
working out where the center is and then observing how the BCG wobbles around
this center.
"We found that that the
BCGs 'slosh' around at the bottom of the halos," says David Harvey.
"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." The wobbling
also shows that BCGs cannot coincide exactly with the cluster's halo, meaning
that certain models of galaxy clusters have to be adjusted.
The scientists will extend
their research with larger surveys of galaxy clusters such as Euclid. They hope
that this will allow them to confirm their findings, but to also determine if
BCG wobbling originates in new fundamental physics or a novel astrophysical
phenomenon.
Via Phys.org