# Ecology of dark matter haloes -II. Effects of interactions on the alignment of halo pairs

## TL;DR

This study uses cosmological simulations to analyze how interactions influence the shape, spin, and orbital alignments of dark matter haloes, revealing evolution patterns over cosmic time and environmental dependence.

## Contribution

It provides new insights into the effects of halo interactions on their physical and orbital alignments across different redshifts and environments.

## Key findings

- Interacting haloes have lower spin and higher sphericity.
- Spin alignments evolve from anti-parallel to parallel over time.
- Alignment signals are stronger at high mass and close separations.

## Abstract

We use the Horizon Run 4 cosmological N -body simulation to study the effects of distant and close interactions on the alignments of the shapes, spins, and orbits of targets haloes with their neighbours, and their dependence on the local density environment and neighbour separation. Interacting targets have a significantly lower spin and higher sphericity and oblateness than all targets. Interacting pairs initially have anti-parallel spins, but the spins develop parallel alignment as time goes on. Neighbours tend to evolve in the plane of rotation of the target, and in the direction of the major axis of prolate haloes. Moreover, interactions are preferentially radial, while pairs with non-radial orbits are preferentially prograde. The alignment signals are stronger at high-mass and for close separations, and independent on the large-scale density. Positive alignment signals are found at redshifts up to 4, and increase with decreasing redshifts. Moreover, the orbits tend to become prograde at low redshift, while no alignment is found at high redshift (z = 4).