Large scale anisotropies on halo infall

TL;DR

This paper analyzes the anisotropic patterns of matter infall onto dark matter haloes and their dependence on halo shape, large-scale surrounding regions, and local density, revealing directional velocity variations and laminar flow tendencies.

Contribution

It provides a detailed statistical characterization of velocity anisotropies around haloes and links these patterns to halo and environment shapes, a novel comprehensive analysis.

Findings

Infall velocity is larger along the halo's minor axis.

Velocity anisotropy is aligned with the principal axes of the large-scale surrounding regions.

Higher local density regions exhibit larger infall velocities.

Abstract

We perform a statistical analysis of the peculiar velocity field around dark matter haloes in numerical simulations. We examine different properties of the infall of material onto haloes and its relation to central halo shapes and the shape of the large scale surrounding regions (LSSR). We find that the amplitude of the infall velocity field along the halo shape minor axis is larger than that along the major axis. This is consistent for general triaxial haloes, and for both prolate and oblate systems. We also report a strong anisotropy of the velocity field along the principal axes of the LSSR. The infall velocity field around dark matter haloes reaches a maximum value along the direction of the minor axis of the LSSR, whereas along the direction of its major axis, it exhibits the smallest velocities. We also analyse the dependence of the matter velocity field on the local environment. The amplitude of the infall velocity at high local density regions is larger than at low local density regions. The velocity field tends to be more laminar along the direction towards the minor axis of the LSSR, where the mean ratio between flow velocity and velocity dispersion is of order unity and nearly constant up to scales of 15 Mpc/h. We also detect anisotropies in the outflowing component of the velocity field, showing a maximum amplitude along the surrounding LSSR major axis.