The Graininess of Dark Matter Haloes

TL;DR

This study uses high-resolution simulations to reveal that dark matter haloes exhibit highly variable local properties and a grainy phase space structure, challenging the assumption of smooth, spherically symmetric models.

Contribution

It provides detailed analysis of local properties and phase space structure of dark matter haloes, highlighting their graininess and deviations from simplified models.

Findings

Local properties vary significantly at the same radius.

Density spans four orders of magnitude in outskirts.

Presence of cold streams and high phase space density regions.

Abstract

We use the recently completed one billion particle Via Lactea II LambdaCDM simulation to investigate local properties like density, mean velocity, velocity dispersion, anisotropy, orientation and shape of the velocity dispersion ellipsoid, as well as structure in velocity space of dark matter haloes. We show that at the same radial distance from the halo centre, these properties can deviate by orders of magnitude from the canonical, spherically averaged values, a variation that can only be partly explained by triaxiality and the presence of subhaloes. The mass density appears smooth in the central relaxed regions but spans four orders of magnitude in the outskirts, both because of the presence of subhaloes as well as of underdense regions and holes in the matter distribution. In the inner regions the local velocity dispersion ellipsoid is aligned with the shape ellipsoid of the halo. This is not true in the outer parts where the orientation becomes more isotropic. The clumpy structure in local velocity space of the outer halo can not be well described by a smooth multivariate normal distribution. Via Lactea II also shows the presence of cold streams made visible by their high 6D phase space density. Generally, the structure of dark matter haloes shows a high degree of graininess in phase space that cannot be described by a smooth distribution function.