Galaxy clusters reveal secrets of dark matter

It has long been inferred by astronomers that the heavier a galaxy cluster is, the more dark matter is in its environment. But new studies are revealing that the relationship between the two could be even more intricate.

Images from the Sloan Digital Sky Survey DR8 show a sparsely populated galaxy cluster on the left and a more densely-packed specimen on the right. A team of astronomers has studied and compared images like these to reveal how dark matter influences the structure of galaxy clusters.
Credit: Sloan Digital Sky Survey

Dark matter accounts for 27 per cent of all matter and energy, yet we cannot see it. However, we can infer its existence because of the effect it has on normal matter in the Universe.

Galaxy clusters make excellent dark matter laboratories for astronomers due to the density of dark matter common in these areas of space.

"Galaxy clusters are like the large cities of our Universe. In the same way that you can look at the lights of a city at night from a plane and infer its size, these clusters give us a sense of the distribution of the dark matter that we can't see," says Hironao Miyatake at NASA's Jet Propulsion Laboratory.

Miyatake has led a new study that suggests dark matter has a bearing on the internal structure of a galaxy cluster, as well as its size.

Researchers looked at about 9,000 galaxy clusters from the Sloan Digital Sky Survey DR8 galaxy catalogue and separated them into two groups: those in which the galaxies were spread out, and those in which the galaxies were more closely packed together.

Using gravitational lensing, which looks at how the gravity of clusters bends light from other objects, the team confirmed both groups had similar masses. But further comparisons between the groups showed that clusters with more closely packed galaxies had fewer neighbouring clusters than the more sparsley packed type. This showed the that surrounding dark matter determines the amount of galaxies in a cluster.

"This difference is a result of the different dark-matter environments in which the groups of clusters formed. Our results indicate that the connection between a galaxy cluster and surrounding dark matter is not characterised solely by cluster mass, but also its formation history," says Miyatake.

"Previous observational studies had shown that the cluster's mass is the most important factor in determining its global properties,” says study co-author David Spergel, professor of astronomy at Princeton University in New Jersey. “Our work has shown that age matters: younger clusters live in different large-scale dark-matter environments than older clusters."

The results of the study also fit with current predictions about the origin and formation of the Universe. Following the Big Bang, an event called cosmic inflation saw small changes occur in the energy in space, leading to the irregular distribution of matter in the Universe. It is thought that these irregularities in the density of matter led to the formation of galaxy clusters.

"The connection between the internal structure of galaxy clusters and the distribution of surrounding dark matter is a consequence of the nature of the initial density fluctuations established before the Universe was even one second old," Miyatake says. "Galaxy clusters are remarkable windows into the mysteries of the Universe. By studying them, we can learn more about the evolution of large-scale structure of the Universe, and its early history, as well as dark matter and dark energy."

Galaxy cluster Abell 1689
Credit: NASA/ESA/JPL-Caltech/Yale/CNRS
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