Dark matter study views wobbling galaxies

The brightest galaxies in galaxy clusters 'wobble', according to a new dark matter study using the Hubble Space Telescope.

The  giant elliptical galaxy in the centre of this image, taken by the  NASA/ESA Hubble Space Telescope, is the most massive and brightest  member of the galaxy cluster Abell 2261. Astronomers refer to it as the brightest cluster galaxy (BCG). Spanning a little over one million light-years, the galaxy is about 20 times the diameter of  our Milky Way galaxy. Astronomers  used Hubble's Advanced Camera for Surveys and Wide Field Camera 3 to  measure the amount of starlight across the galaxy, catalogued as 2MASX  J17222717+3207571 but more commonly called A2261-BCG (short for Abell  2261 brightest cluster galaxy). Abell 2261 is located three billion  light-years away. The  observations were taken between March and May 2011. The Abell 2261  cluster is part of a multi-wavelength survey called the Cluster Lensing  And Supernova survey with Hubble (CLASH).

At the centre of this image is an elliptical galaxy, the brightest member of galaxy cluster Abell 2261. A study showing that galaxies like these wobble could shake up our current understanding of dark matter. Credit: NASA, ESA, M. Postman (Space Telescope Science Institute, USA), T. Lauer (National Optical Astronomy Observatory, USA), and the CLASH team.

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A study of galaxy clusters has revealed that their brightest galaxies ‘wobble’, which could have implications for the current understanding of dark matter.

Galaxy clusters are collections of up to a thousand galaxies that often form as a result of violent mergers.

At the centre of galaxy clusters lies a dense core that contains a massive galaxy, known as the brightest cluster galaxy (BCG).

Once a galaxy cluster returns to a relaxed state after a merger, it is thought that the BCG is held in place by the gravitational influence of dark matter.

Dark matter is an invisible substance thought to make up over 25 per cent of all matter in the Universe.

It cannot be directly observed, but scientists can infer its existence from the effect it has on the visible matter in the Universe.

A team of astronomers used the Hubble Space Telescope to study ten different galaxy clusters.

They found that the BGCs were not fixed at the centre, as expected.

In fact, Hubble data revealed that the centre of the visible parts of each cluster and the centre of the total mass of the cluster were offset by as much as 40,000 lightyears.

The team behind the study were able to analyse the galaxy clusters because they also act as gravitational lenses.

This means that they are so massive, they warp spacetime.

Light from distant objects is distorted as it passes through the clusters, and this effect can be used to make a map of the dark matter associated with the cluster.

The swirls of light surrounding giant galaxy cluster Abell 383 at the centre of this image show gravitational lensing in action: the effects of massive objects on space-time.Credit: NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM). Acknowledgement: Marc Postman (STScI)
The swirls of light surrounding giant galaxy cluster Abell 383 at the centre of this image show gravitational lensing in action: the effects of massive objects on space-time. Credit: NASA, ESA, J. Richard (CRAL) and J.-P. Kneib (LAM). Acknowledgement: Marc Postman (STScI)

Scientists can then work out the centre of mass and thus calculate whether the BGC is offset from the centre.

If the wobbling spotted during the study is a result of the effects of dark matter, it could only be explained if dark matter particles can interact with each other.

This would cause a rethink on the current understanding of dark matter.

Co-author of the study Frederic Courbin at EPFL in Switzerland says:

“We’re looking forward to larger surveys — such as the Euclid survey — that will extend our dataset.

Then we can determine whether the wobbling of BGCs is the result of a novel astrophysical phenomenon or new fundamental physics.

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Both of which would be exciting!”