Evolution of the Phase-Space Density in Dark Matter Halos

• Published in: The Astrophysical journal Vol. 671; no. 2; pp. 1108 - 1114
• Main Authors: Hoffman, Yehuda, Romano-Díaz, Emilio, Shlosman, Isaac, Heller, Clayton
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 20.12.2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Galaxy evolution; Galaxy halo; Galaxies; galaxies: evolution; galaxies: interactions; Phase space density; galaxies: halos; galaxies: formation; Dark matter; Dynamics; Galaxy formation; Kinematics; galaxies: kinematics and dynamics
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/523695

The evolution of the phase-space density profile in dark matter (DM) halos is investigated by means of constrained simulations, designed to control the merging history of a given DM halo. Halos evolve through a series of quiescent phases of a slow accretion intermitted by violent events of major mergers. In the quiescent phases the density of the halo closely follows the NFW profile and the phase-space density profile, Q(r), is given by the Taylor & Navarro power law, r super(- beta ), where [unk] approximately 1.9 and stays remarkably stable over the Hubble time. Expressing the phase-space density by the NFW parameters, Q(r) = Q sub(s)(r/R sub(s)) super(- beta ), the evolution of Q is determined by Q sub(s). We have found that the effective mass surface density within R sub(S), [unk] identical with [unk], remains constant throughout the evolution of a given DM halo along the main branch of its merging tree. This invariance entails that Q sub(s) proportional to [unk] and Q(r) proportional to [unk] [unk] [unk]. It follows that the phase-space density remains constant, in the sense of Q sub(s) = const., in the quiescent phases and it decreases as R super(-) sub(s) super(5/2) in the violent ones. The physical origin of the NFW density profile and the phase-space density power law is still unknown. Yet, the numerical experiments show that halos recover these relations after the violent phases. The major mergers drive R sub(s) to increase and Q sub(s) to decrease discontinuously while keeping Q sub(s) x R super(5) sub(s) super(/2) = const. The virlal equilibrium in the quiescent phases implies that a DM halos evolves along a sequence of NFW profiles with constant energy per unit volume (i.e., pressure) within R sub(s).