These are density maps of galaxy cluster distribution. Credit: Kavli IPMU
First observational evidence for assembly bias could impact understanding of the universe. An international team of researchers has shown that the relationship between galaxy clusters and their surrounding dark matter halo is more complex than previously thought. The researchers’ findings are the first to use observational data to show that, in addition to mass, a galaxy cluster’s formation history plays a role in how it interacts with its environment.
There is a connection between galaxy clusters and their dark matter halos that holds a great deal of information about the universe’s content of dark matter and accelerating expansion due to dark energy. Galaxy clusters are groupings of hundreds to thousands of galaxies bound together by gravity, and are the most massive structures found in the universe. These clusters are embedded in a halo of invisible dark matter. Traditionally, cosmologists have predicted and interpreted clustering by calculating just the masses of the clusters and their halos. However, theoretical studies and cosmological simulations suggested that mass is not the only element at play – something called assembly bias, which takes into account when and how a galaxy cluster formed, also could impact clustering.
“Simulations have shown us that assembly bias should be part of our picture,” said Mandelbaum, McWilliams Center for Cosmology. “Confirming this observationally is an important piece of understanding galaxy and galaxy cluster formation and evolution.” The research team, led by Miyatake, More and Takada of the Kavli Institute for the Physics and Mathematics of the Universe, analyzed observational data from the Sloan Digital Sky Survey’s DR8 galaxy catalog. They demonstrated that when and where galaxies group together within a cluster impacts the cluster’s relationship with its dark matter environment.
The researchers divided close to 9,000 galaxy clusters into 2 groups based on the spatial distribution of the galaxies in each cluster. One group consisted of clusters with galaxies aggregated at the center and the other consisted of clusters in which the galaxies were more diffuse. They then used gravitational lensing to show that, while the two groups of clusters had the same mass, they interacted with their environment much differently. The group of clusters with diffuse galaxies were much more clumpy than the group of clusters that had their galaxies close to the center.
“Measuring the way galaxy clusters clump together on large scales is a linchpin of modern cosmology. We can go forward knowing that mass might not be the only factor in clustering,” Mandelbaum said. http://www.cmu.edu/news/stories/archives/2016/january/galaxy-clusters.html
Recent Comments