okt 092012


Scientists have used a suite of X-ray observations to find evidence of large scale swirling gas motion in the Perseus galaxy cluster.

The Perseus Cluster, located 237 million light-years away in the constellation of Perseus, contains thousands of galaxies immersed in a vast cloud of multimillion degree gas. Image Credit: Jean-Charles Cuillandre (CFHT) & Giovanni Anselmi (Coelum Astronomia), Hawaiian Starlight

Clusters of galaxies are the largest gravitationally bound objects in existence, containing hundreds or thousands of galaxies, most of the gas in the Universe, and enormous quantities of dark matter, all held together by the collective gravitational pull of all that mass. As such, they are important laboratories for astrophysics and cosmology, and understanding what goes on in clusters is key to investigations of dark matter and dark energy, as well as galaxy formation and the interaction of galaxies with their environment.

The gas in clusters is super-heated through shocks as it falls towards the center, and so glows in X-rays. X-ray astronomers, including the KIPAC group led by Professor Steve Allen, have become experts at combing maps of clusters in this light to understand the density, temperature, and entropy of the gas throughout, which has helped build a coherent picture of cluster dynamics and enabled their use for cosmological measurements.

There have been hints indicating large scale spiral motions of the gas near the centers of clusters, where the gas is denser and cooler. The motions manifest in distinctively brighter thin ‘fronts’ that divide relatively denser gas from a relatively less dense area, like a front on a weather map. Now, an investigation by a team of astrophysicists has concluded, somewhat surprisingly, that for the well studied Perseus galaxy cluster, the motions extend throughout the whole cluster, to a great distance from the center, where models have assumed that large scale motions would not be present.

Image of the Perseus cluster showing the ratio of the X-ray surface brightness to the average surface brightness at that distance from the center of the cluster for ROSAT and XMM-Newton satellite observations. Curved ‘fronts’ of brightness are evident. Image Credit: KIPAC. For a larger image, click here.

The team, led by KIPAC postdoc Aurora Simionescu,  combed through large scale mosaic X-ray images of the Perseus cluster from the ROSAT, XMM-Newton, Chandra, and Suzaku X-ray observing satellite telescopes.

They found that fronts curve from the center of the cluster out toward the very edges, indicating a churning or sloshing motion that extends throughout the cluster. This is in contrast to recent computer simulations which attempted to reconstruct the gas dynamics of the cluster from what is known about its other properties, which predicted that such motions should be much more confined toward the center. In the computer simulations, where the gas is allowed to evolve according to the known physics, the central sloshing motions result from the after-effects of minor mergers, where clusters and sub-components of them interacted and jostled material in the past.

It is an open question, then, what causes the large-scale gas churning in Perseus reported by the team. It is possible that different specific merger characteristics could result in a simulation result more like the observed ones, and this could be investigated through further simulations. Alternately, our understanding of the cluster’s gas properties, such as its viscosity, may need to be revised.

It is also possible that the observed motions are the result of a major merger that happened in the past, where Perseus and another galaxy cluster collided directly. If such a major merger did in fact occur, it did surprisingly little disturbance to the center of the Perseus cluster, where the gas has evidently been steadily cooling through normal interactions among the particles for a very long time. It is clear that these newly discovered large-scale motions present another intriguing question about the giants of the Universe.

This work is described in a paper published in The Astrophysical Journal (2012, ApJ, 757, 182).

Source: Kavli Institute for Particle Astrophysics and Cosmology (KIPAC)

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