Dynamics of pairwise motions in the Cosmic Web

TL;DR

This paper analyzes how dark matter pairwise velocities vary across different Cosmic Web environments, revealing environment-dependent dynamics consistent with gravitational instability, and suggests observational tests for Cosmic Web segmentation algorithms.

Contribution

It introduces a method to classify Cosmic Web environments and analyzes their impact on dark matter pairwise velocities using simulation data, highlighting environment-dependent dynamics.

Findings

Pairwise velocities differ significantly across environments.

Walls and voids show colder, slower pairwise flows.

Results align with gravitational instability predictions.

Abstract

We present results of analysis of the dark matter (DM) pairwise velocity statistics in different Cosmic Web environments. We use the DM velocity and density field from the Millennium 2 simulation together with the NEXUS+ algorithm to segment the simulation volume into voxels uniquely identifying one of the four possible environments: nodes, filaments, walls or cosmic voids. We show that the PDFs of the mean infall velocities $v_{12}$ as well as its spatial dependence together with the perpendicular and parallel velocity dispersions bear a significant signal of the large-scale structure environment in which DM particle pairs are embedded. The pairwise flows are notably colder and have smaller mean magnitude in wall and voids, when compared to much denser environments of filaments and nodes. We discuss on our results, indicating that they are consistent with a simple theoretical predictions for pairwise motions as induced by gravitational instability mechanism. Our results indicate that the Cosmic Web elements are coherent dynamical entities rather than just temporal geometrical associations. In addition it should be possible to observationally test various Cosmic Web finding algorithms by segmenting available peculiar velocity data and studying resulting pairwise velocity statistics