An attractor for dark matter structures

Abstract

Cosmological simulations of dark matter structures have identified a set of universal profiles, and similar characteristics have been seen in non-cosmological simulations. It has therefore been speculated whether these profiles of collisionless systems relate to accretion and merger history, or if there is an attractor for the dark matter systems. Here we identify such a 1-dimensional attractor in the 3-dimensional space spanned by the 2 radial slopes of the density and velocity dispersion, and the velocity anisotropy. This attractor effectively removes one degree of freedom from the Jeans equation. It also allows us to speculate on a new fluid interpretation for the Jeans equation, with an effective polytropic index for the dark matter particles between 1/2 and 3/4. If this attractor solution holds for other collisionless structures, then it may hold the key to break the mass-anisotropy degeneracy, which presently prevents us from measuring the mass profiles in dwarf galaxies uniquely.

7 pages, 2 figures, comments welcome

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• Physical properties underlying observed kinematics of satellite galaxies
• The structure and assembly history of cluster-size haloes in Self-Interacting Dark Matter
• Orbital anisotropy in cosmological haloes revisited
• Statistical mechanics of collisionless orbits. III. Comparison with N-body simulations
• Relaxation of N-body systems with additive r^-α interparticle forces
• Self-gravitating Equilibria of Non-minimally Coupled Dark Matter Halos
• The temperature of hot gas in galaxies and clusters: baryons dancing to the tune of dark matter
• Particle ejection during mergers of dark matter halos
• Stirring N-body systems: Universality of end states
• A derivation of (half) the dark matter distribution function
• An attractor for the dynamical state of the intracluster medium
• Stirring N-body systems II: Necessary conditions for the dark matter attractor