Orbital anisotropy in cosmological haloes revisited

Understand

The velocity anisotropy of particles inside dark matter (DM) haloes is an important physical quantity, which is required for the accurate modelling of mass profiles of galaxies and clusters of galaxies.

• It is typically measured using the ratio of the radial-to-tangential velocity dispersions at a given distance from the halo centre.
• However, this measure is insufficient to describe the dynamics of realistic haloes, which are typically quite elongated.
• Studying the velocity distribution in massive DM haloes in cosmological simulations, we find that in the inner parts of the haloes the local velocity ellipsoids are strongly aligned with the major axis of the halo, the alignment being stronger for more relaxed haloes.