The Asymptotic Form of Cosmic Structure: Small‐Scale Power and Accretion History

Abstract

We explore the effects of small-scale structure on the formation and equilibrium of dark matter halos in a universe dominated by vacuum energy. We present the results of a suite of four N-body simulations, two with a ΛCDM initial power spectrum and two with WDM-like spectra that suppress the early formation of small structures. All simulations are run into the far future when the universe is 64 h-1 Gyr old, long enough for halos to essentially reach dynamical equilibrium. We quantify the importance of hierarchical merging on the halo mass accretion history, the substructure population, and the equilibrium density profile. We modify the mass accretion history function of Wechsler and coworkers by introducing a parameter, γ, that controls the rate of mass accretion, d ln M/d ln a ∝ a-γ, and find that this form characterizes both hierarchical and monolithic formation. Subhalo decay rates are exponential in time with a much shorter timescale for WDM halos. At the end of the simulations, we find truncated Hernquist density profiles for halos in both the CDM and WDM cosmologies. There is a systematic shift to lower concentration for WDM halos, but both cosmologies lie on the same locus relating concentration and formation epoch. Because the form of the density profile remains unchanged, our results indicate that the equilibrium halo density profile is set independently of the halo formation process.