Measuring Cluster Relaxedness
When is a dark matter halo 'relaxed'? In our efforts to understand the structure of the universe, dark matter simulations have provided essential grounds for theoretical predictions. These simulations provide a wealth of ways of parameterizing and measuring the features of astronomical objects. It is these measurements on which we base comparisons of our world and our attempts to re-create it. One of the essential questions dark matter simulations help address is how dark matter halos evolve. How does one characterize different states of that evolution? The focus of this project is identifying cluster relaxedness and how it relates to the internal structure of the halo. A dark matter simulation consists of an N-body simulation which takes an initial set of positions and velocities of the dark matter particles and evolves them under the influence of gravity [6]. Though scientists have so far not been able to detect dark matter particles, the information from these simulations is still valuable especially given the relationship between dark matter halos and galaxy clusters. Galaxies sit within dark matter halos and recent evidence points to filaments of dark matter forming the framework on which galaxy clusters grow [7]. A dark matter halo is a collapsed group of gravitationally bound dark matter particles. Subsets of bound particles form subhalos or substructures. The dark matter simulation is carried out over time - with decreasing redshift (z) or increasing scale factor (a = 1/1+z ). (Thus, z = 0 or a = 1.0 is present-day.) The merger history of a halo can be represented pictorally by a merger tree. A major merger event occurs when a structure joins the main halo with the mass ratio between it and the main halo being above a certain threshold. These events mark important points in the halo's evolution. And it is at these events that one hopes, and perhaps is more likely, to relate measures of relaxedness to this mass accretion. Cluster relaxedness is not a well-defined concept. Rather a set of qualities are defined that one expects a 'relaxed' cluster to have. One expects a relaxed halo to have a roughly isotropic density distribution. Most of the particles should be part of the main halo rather than bound in substructures. Taking into account kinematic information of the halo, one does not expect a proportion of the particles energy in kinetic energy that goes far beyond virial equilibrium. With our measures of cluster relaxedness, we want to investigate its relationship to these major merger events. We'll first look at how measurements of different aspects of relaxedness relate to each other and to other aspects of a halo's internal structure. We'll then look at how these measurements behave in response to major mergers.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 1049742
- Report Number(s):
- SLAC-TN-12-011; TRN: US201218%%603
- Country of Publication:
- United States
- Language:
- English
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