The Equilibrium Structure of Dark Matter Halos.

The far future provides an excellent laboratory for studying cosmological structures. The eventual dominance of the cosmological constant causes the universe to enter a phase of exponential deSitter expansion during which dark matter halos are allowed to relax without interruptions from mass accretion. Using of suite of N-body simulations evolved to scale factor a = 100 in an LCDM universe, this thesis presents the equilibrium distribution and structure of halos and investigates the importance of mergers in setting the internal halo structure. Using conservation of energy, it is possible to accurately predict the total amount of mass that will ultimately accrete onto a halo. Once this mass is accreted, the halo quickly reaches a state of dynamical equilibrium. Halos in this equilibrium have a greatly simplified radial phase space profile characterized by a single zero-velocity surface that unambiguously defines the halo edge. The radial density profile for such halos is well fit by an NFW profile inside r200, but is steeper at larger radii and better fit by a truncated Hernquist profile. In order to study the importance of hierarchical merging in setting the equilibrium structure, I also present results form LWDM-like simulations with initial power spectra that suppress the early formation of small halos. Using these simulations, I present a modified fit to the mass accretion form of Wechsler et al. (2002) that better characterizes halo growth at all epochs. At the end of the simulations, we recover density profiles and phase space structures that are virtually unchanged between the CDM and WDM cosmologies. The only difference in the global halo properties, a systematic concentration shift, can be characterized in terms of the halo formation epoch. We conclude that mass accretion is not the driving process for setting the equilibrium halo structure.