Exploring the Dynamics of Galaxy Cluster Mergers

We are Tessa Thorsen and Andre Hinds, rising Juniors in the Physics Department at Gettysburg College. This summer we worked with Dr. Johnson analyzing galaxy cluster mergers. Galaxy clusters are gravitationally bound groupings of large numbers of galaxies and smaller subgroups, leading to the presence of complex substructure within the cluster.


Abell 426 – Perseus Galaxy Cluster. Source: universetoday.com

There are many theories on how these clusters form, so looking at the merger process can give us insight as to which of these theories might be most applicable. Galaxies, and the clusters they form, are actually extremely sparse, so during a merger the only collisional matter is the interstellar gas. For our research we are not looking at this gas. Rather, we are examining what happens to the non-collisional matter that makes up the galaxies. Our goal is to determine statistical techniques for deciding which galaxies belong to which clusters, finding substructure in the clusters, and describing the dynamics of the clusters at different stages in the merger.

The video below is a visualization of simulated galaxy cluster mergers. The first 50 seconds of the video show how particles in the two different clusters, yellow and blue, interact throughout the merger. The rest of the video shows the collisional gas during the merger, which is interesting, but not our focus.

We have been looking at the same data presented in the video, which was created by Dr. Zuhone, one of Dr. Johnson’s collaborators at the Goddard Space Flight Center. The data contains nine different simulations, each with different initial parameters representing the mass ratios of the clusters and the angle of approach between the two clusters. Each simulation contains 100,000 particles, which are representative of the dark matter present in the clusters.

We are looking at simulated data rather than observational data because it allows us to manipulate time and space in a way that is impossible when performing observations through a telescope. With the simulated data we can see positional and velocity data in three dimensions, we can observe the entire merger throughout periods of hundreds of millions of years, and we have a priori knowledge of which galaxies belong to which clusters. This means that we have a better idea of the accuracy of any tests we perform.

From here we have split the project up into two parts. The first part is concerned with determining cluster membership of galaxies throughout the merger. We have attempted to apply a partitioning algorithm developed by mathematicians and other researchers in this field, and have compared the results to the known cluster distributions. It is fairly simple to determine membership at the beginning of the merger, but as the clusters begin to coalesce it is harder to distinguish between them. The algorithm requires fairly exact estimates of  the average velocity and position of each cluster, so we need to determine a better method for approximating these measurements, without prior knowledge.

The second part focuses on describing how the clusters behave throughout the merger. We are projecting the three dimensional data we have onto a plane, representing the plane of the sky, because it allows us to see the simulated data as we would see observational data. From there, we have looked at the change in the central position and scale of the galaxies throughout the merger, as well as the velocity dispersion. The velocity dispersion is a measure of the difference in velocity of the particles throughout the cluster, and is a very helpful statistic for comparing the dynamics of clusters. We have found that, as the clusters approach each other, their velocity dispersions increase dramatically, as particles begin to feel more intense gravitational attraction, and then flatten out as the merger progresses.

Graph of velocity dispersion and position of two clusters undergoing a merger.

Graph of velocity dispersion and position of two clusters undergoing a merger.

Studying galaxy clusters and their substructure can illuminate many of the mysteries behind the formation of our universe. Substructure can tell us how quickly our universe was and is expanding, how matter was initially distributed throughout space, and even how old our universe is. Much research has been done on this subject, and many of these things are becoming increasingly well known, but research such as ours comes at it from a different angle, and can provide a unique perspective.


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