Clarifying the Role of Thermodynamics in Self-gravitating Dark Matter Systems

Abstract

Astrophysicists currently favor the idea that the vast majority of mass in the uni- verse exists in a collisionless form that interacts only through gravity. This paradigm has been successful in allowing computer simulations of cosmological volumes to re- produce structures similar to those observed. Our work aims to better understand the physics at work in collisionless systems, in general. Specifically, we investigate how well thermodynamics-based approaches to understanding equilibrium structures agree with results of computer simulations. Using a suite of N -body simulations with differ- ing initial conditions, we examine the density and velocity profiles of self-gravitating collisionless systems. Statistical comparisons between simulated equilibrium struc- tures and two thermodynamics-based models indicate the relative appropriateness of the models. We find that no single model can describe systems resulting from the entire range of initial conditions investigated here. Our major result is that these thermodynamics-based models can successfully reproduce equilibria that arise in gen- tly evolving systems. However, the failure of thermodynamics-based models to de- scribe the equilibria of more violent evolutions suggests that they must retain some memory of their initial conditions.