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In galaxy clustering, mass may not be the only thing that matters

First observational evidence for assembly bias could impact understanding of the universe

Date:
January 25, 2016
Source:
Carnegie Mellon University
Summary:
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.
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These are density maps of galaxy cluster distribution.
Credit: Kavli IPMU

An international team of researchers, including Carnegie Mellon University's Rachel Mandelbaum, has shown that the relationship between galaxy clusters and their surrounding dark matter halo is more complex than previously thought. The researchers' findings, published in Physical Review Letters today (Jan. 25), 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, a member of Carnegie Mellon's McWilliams Center for Cosmology. "Confirming this observationally is an important piece of understanding galaxy and galaxy cluster formation and evolution."

In the current study, the research team, led by Hironao Miyatake, Surhud More and Masahiro 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. Using this data, 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 two 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 a technique called 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.


Story Source:

The above post is reprinted from materials provided by Carnegie Mellon University. The original item was written by Jocelyn Duffy. Note: Materials may be edited for content and length.


Journal Reference:

  1. Hironao Miyatake, Surhud More, Masahiro Takada, David N. Spergel, Rachel Mandelbaum, Eli S. Rykoff, and Eduardo Rozo. Evidence of Halo Assembly Bias in Massive Clusters. Phys. Rev. Lett., 25 January 2016 DOI: 10.1103/PhysRevLett.116.041301

Cite This Page:

Carnegie Mellon University. "In galaxy clustering, mass may not be the only thing that matters: First observational evidence for assembly bias could impact understanding of the universe." ScienceDaily. ScienceDaily, 25 January 2016. <www.sciencedaily.com/releases/2016/01/160125114233.htm>.
Carnegie Mellon University. (2016, January 25). In galaxy clustering, mass may not be the only thing that matters: First observational evidence for assembly bias could impact understanding of the universe. ScienceDaily. Retrieved July 27, 2016 from www.sciencedaily.com/releases/2016/01/160125114233.htm
Carnegie Mellon University. "In galaxy clustering, mass may not be the only thing that matters: First observational evidence for assembly bias could impact understanding of the universe." ScienceDaily. www.sciencedaily.com/releases/2016/01/160125114233.htm (accessed July 27, 2016).

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