An attractor for dark matter structures

TL;DR

This paper identifies a one-dimensional attractor in dark matter structures that simplifies the understanding of their density and velocity profiles, potentially resolving the mass-anisotropy degeneracy in dwarf galaxy observations.

Contribution

The discovery of a 1D attractor in the phase space of dark matter structures introduces a new way to interpret their profiles and suggests a fluid-like model for collisionless systems.

Findings

Identified a 1D attractor in the phase space of dark matter systems.

The attractor reduces the degrees of freedom in the Jeans equation.

Proposed a fluid interpretation with an effective polytropic index.

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.