Back to square one on dark matter find


Better imaging reveals earlier observation to be a product of less-than-perfect optics. Andrew Masterson reports.


A view of the four central galaxies at the heart of cluster Abell 3827, combining Hubble Space Telescope imaging in the ultraviolet (shown as blue), and Atacama Large Millimetre Array imaging (shown as red contour lines).
A view of the four central galaxies at the heart of cluster Abell 3827, combining Hubble Space Telescope imaging in the ultraviolet (shown as blue), and Atacama Large Millimetre Array imaging (shown as red contour lines).
NASA/ESA/ESO/Richard Massey (Durham University)

It was an exciting discovery while it lasted, but fresh observations show that evidence for a valuable insight into the nature of dark matter was, quite literally, a trick of the light.

Three years ago, a team of cosmologists, primarily from Durham University in the UK, reported that observations from the Hubble Telescope revealed a galaxy that had become separated from its own dark matter.

The galaxy was in a cluster known as Abell 3827, about 1.3 billion light years from Earth. The team, led by Richard Massey of the university’s Centre for Extragalactic Astronomy, looked at four galaxies in the group. Each retained a dark matter halo, but in one it was significantly offset from its stars.

This was an exciting find. The separation of dark matter from “normal” visible matter is predicted in cosmology – but only if dark matter interacts with additional forces as well as gravity. The Abell 3827 data seemed to provide a strong indication that this was in fact the case, leading scientists a little bit closer to understanding the nature of the mysterious substance that is estimated to account for 27% of the universe.

Alas, in evidence presented to the European Week of Astronomy and Space Science just concluded in the English city of Liverpool, the same group of researchers report that a more recent set of observations have shown that the separation of dark matter from the galaxy stars was an artefact produced by the relative orientations of Abell 3827 and the Hubble.

Massey told attendees at the event that new observations made by the Atacama Large Millimetre Array (ALMA) in Chile, South America, revealed the offset dark matter was, in fact, not offset at all.

The ALMA facility happened to pick up distorted infrared light from a galaxy situated a long way behind the target. This resulted in a higher resolution image than the ones obtained by the Hubble – revealing that the apparently absent dark matter had been there, undetected, all the time.

“The search for dark matter is frustrating, but that's science,” says Massey. “When data improves, the conclusions can change. Meanwhile the hunt goes on for dark matter to reveal its nature.

“So long as dark matter doesn't interact with the universe around it, we are having a hard time working out what it is.”

The team’s findings will be published in the journal Monthly Notices of the Royal Astronomical Society. In the meantime, a preprint version from the site Arxiv can be found here.

  1. https://academic.oup.com/mnras/article-abstract/449/4/3393/1171981
  2. http://www.gemini.edu/gallery/media/abell-3827
  3. http://eas.unige.ch/EWASS2018/
  4. https://arxiv.org/abs/1708.04245
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