Galaxy Clusters and Dark Matter

Scientists have found that dark matter resides in regions of space where matter is dense, which is why galaxy clusters (huge clumps of stars and planets) are vital to learning about the mysterious phenomenon that makes up more than 20 percent of all matter and energy in the universe.

Because dark matter is invisible (despite the fact that it is universally present), scientists must infer its existence by observing how it affects regular matter. They do this based on the idea that, the heavier and denser a cluster is, the more dark matter is in that area, but a new study suggests a more complicated connection.

Hironao Miyatake from NASA’s Jet Propulsion Laboratory in Pasadena California says that “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.”

In their new study, Miyatake and his team suggest that a galaxy cluster’s internal structure is somehow linked to the the dark matter that resides around it, making it the first time that something other that a cluster’s mass has been linked to the dark matter surrounding it.

Their study was published in Physical Review Letters.

Estimation of dark matter, dark energy, and normal matter. Credit: NASA
Looking at the Bigger Picture

To make the connection, the researchers studied about 9,000 galaxy clusters with help of the Sloan Digital Sky Survey DR8 galaxy catalog. From there, they divided these clusters into two groups based on their internal structures.

In one group, galaxies were more spread out within the cluster, while in the other group galaxies were more densely packed. To help with this cataloging, they used a technique called gravitational lensing, which lets them look at how gravity in the clusters bend light.

They found that in the denser cluster group had fewer neighboring clusters tha the less dense cluster group. This means, according to the team, that dark matter determines how densely packed galaxies within a cluster are.

Miyatake said “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 characterized solely by cluster mass, but also its formation history.”

The researchers say that this connection is a “consequence of the nature of the initial density fluctuations established before the universe was even one second old.”

The research team will continue exploring the connection between galaxy clusters and dark matter.

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