Artist's impression of a dwarf galaxy seen from the surface of a hypothetical exoplanet.

If a galaxy is seen as a peach, the standard cosmological model portrays dark matter as the ‘pit at its centre’. Now a study of two dwarf galaxies by astrophysicists at the Institute of Astronomy suggests that dark matter is evenly distributed to make a ‘pitless peach’.

The researchers’ data showed that in the case of both dwarf galaxies, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across.

Like all galaxies, our Milky Way is home to a strange substance called dark matter. Dark matter is invisible, betraying its presence only through its gravitational pull. Without dark matter holding them together, our galaxy’s speedy stars would fly off in all directions. The nature of dark matter is a mystery – a mystery that a new study has only deepened.

Research carried out by a team of astrophysicists at the Cambridge University Institute of Astronomy challenges current understanding of the properties of dark matter on very small scales. Published in The Astrophysical Journal, the work is leading to a re-examination of the predictions made by the standard dark matter model.

“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, who is now a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics in the USA. “We now have even more questions to answer – such as why the current cosmological paradigm fails to describe the distribution of dark matter in two of the most dark-matter dominated galaxies of the universe.”

The standard cosmological model describes a universe dominated by dark energy and dark matter. Most astronomers assume that dark matter consists of “cold” (ie slow-moving) exotic particles that clump together gravitationally. Over time these dark matter clumps grow and attract normal matter, forming the galaxies we see today.

Cosmologists use powerful computers to simulate this process. Their simulations show that dark matter should be densely packed in the centers of galaxies. Instead, new measurements of two dwarf galaxies show that they contain a smooth distribution of dark matter. This suggests that the standard cosmological model may be wrong.

“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” Walker said.

Dwarf galaxies are composed of up to 99 percent dark matter and only one percent normal matter like stars. This disparity makes dwarf galaxies ideal targets for astronomers seeking to understand dark matter.

Walker and his co-author Jorge Peñarrubia (who is now a Ramon y Cajal Fellow at the Instituto de Astrofisica de Andalucia in Spain) analysed the dark matter distribution in two Milky Way neighbours: the Fornax and Sculptor dwarf galaxies. These galaxies hold one million to 10 million stars, compared with about 400 billion in our galaxy. Using the Magellan telescopes at the Las Campanas Observatory in Chile, the researchers measured the locations, speeds and basic chemical compositions of 1,500 to 2,500 stars.

“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” explained Peñarrubia. “That makes it much more challenging to determine the distribution of dark matter. To meet these challenges we chose to look at the two Milky Way galaxies for which we have the most spectroscopic coverage.”

The researchers’ data showed that in the case of both dwarf galaxies, the dark matter is distributed uniformly over a relatively large region, several hundred light-years across. This contradicts the prediction that the density of dark matter should increase sharply toward the centres of these galaxies.

“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit at the centre’. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Peñarrubia.

Some have suggested that interactions between normal and dark matter could spread out the dark matter, but current simulations do not indicate that this happens in dwarf galaxies. The new measurements imply that either normal matter affects dark matter more than expected, or that dark matter is not “cold.”

The researchers now hope to determine which is true by studying more dwarf galaxies, particularly galaxies with an even higher percentage of dark matter. “If we are able to show that our results from the two dwarf galaxies extend to fainter, even more dark-matter dominated galaxies, then a revision of the current matter paradigm would be in order,” said Peñarrubia.

 


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