Anatomy of a Merger: Understanding the Dynamics of Galaxy Cluster Mergers

University Of California-Davis, Davis CA

Investigators

Abstract

The team will use observational data, from telescopes on the ground and in space, to study how particles of dark matter interact with each other. Much of the mass of the Universe is in the form of non-radiating, thus "dark", matter. Particles of dark matter respond to the force of gravity, but little else is known about them. An important, basic step is to measure how strongly dark-matter particles interact with each other, independent of gravity. When physicists want to investigate the basic properties of sub-atomic particles, they smash them together in expensive man-made colliders. Fortunately, Nature has provided dark-matter colliders, for free, in the form of merging clusters of galaxies. The team will analyze the "skid marks" of a few dozen cluster collisions and use the results to study how the dark-matter particles interact with each other, independent of gravity. For broader impacts, the PI has noted that students in four-year colleges can experience an information gap regarding the practicalities of applying to PhD programs. In an attempt to close this gap, the PI will continue running his popular Graduate Physics Admission Boot Camp that serves institutions in northern California. The aim of the proposal is to combine multi-wavelength observations and simulations of an ensemble of merging galaxy clusters to constrain the "self-interaction cross-section" of dark-matter particles. The merging clusters can be thought of as dark-matter colliders. Gravitational lensing is used as a tool to map out the distribution of the dark-matter particles. Cluster membership data are used to age-date the collision. Given this information, simulations can be used to constrain how strongly the dark-matter particles need to interact with each other (the "self-interaction cross-section") to reproduce the snapshot of the collision. This technique becomes stronger if an ensemble of merging clusters is studied, instead of only one. The proposal will apply the technique to an ensemble of merging clusters. Moreover, the proposal also takes the novel step of requiring that the mergers exhibit radio relics. This requirement is important because it provides extra, independent information about the merger ages and also ensures the plane-of-sky viewing angles that will maximize the effects being sought.