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Title:
On Relaxation Processes in Collisionless Mergers
Authors:
Valluri, Monica; Vass, Ileana M.; Kazantzidis, Stelios; Kravtsov, Andrey V.; Bohn, Courtlandt L.
Affiliation:
AA(Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637; .; Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637.), AB(Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637.; Department of Astronomy, University of Florida, Gainesville, FL 32611.), AC(Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637; .; Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637.), AD(Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637; .; Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637.; Enrico Fermi Institute, University of Chicago, Chicago, IL 60637.), AE(Department of Physics, Northern Illinois University, DeKalb, IL 60115.)
Publication:
The Astrophysical Journal, Volume 658, Issue 2, pp. 731-747. (ApJ Homepage)
Publication Date:
04/2007
Origin:
UCP
ApJ Keywords:
Cosmology: Theory, Cosmology: Dark Matter, Methods: Numerical
DOI:
10.1086/511298
Bibliographic Code:
2007ApJ...658..731V

Abstract

We analyze N-body simulations of halo mergers to investigate the mechanisms responsible for driving mixing in phase space and the evolution to dynamical equilibrium. We focus on mixing in energy and angular momentum and show that mixing occurs in a steplike fashion following pericenter passages of the halos. This makes mixing during a merger unlike other well-known mixing processes such as phase mixing and chaotic mixing, whose rates scale with local dynamical time. We conclude that the mixing process that drives the system to equilibrium is primarily a response to energy and angular momentum redistribution that occurs due to impulsive tidal shocking and dynamical friction rather than a result of chaotic mixing in a changing potential. We also analyze the merger remnants to determine the degree of mixing at various radii by monitoring changes in radius, energy, and angular momentum of particles. We confirm previous findings that show that the majority of particles retain strong memory of their original kinetic energies and angular momenta, but do experience changes in their potential energies owing to the tidal shocks they experience during pericenter passages. Finally, we show that a significant fraction of mass (~40%) in the merger remnant lies outside its formal virial radius, and that this matter is ejected roughly uniformly from all radii outside the inner regions. This highlights the fact that mass, in its standard virial definition, is not additive in mergers. We discuss the implications of these results for our understanding of relaxation in collisionless dynamical systems.
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