The Hierarchical Nature of the Spin Alignment of Dark Matter Haloes in Filaments

TL;DR

This paper investigates how the spin alignment of dark matter haloes with cosmic structures varies with hierarchical level in the Cosmic Web, revealing a mass-dependent transition influenced by the web's hierarchy.

Contribution

It introduces MMF-2, an improved method for hierarchical cosmic structure identification, and links halo spin alignment to hierarchical levels in the Cosmic Web.

Findings

Halo spin alignment depends on hierarchical level in the Cosmic Web.

Transition mass for alignment change varies with hierarchy.

Hierarchical structure influences angular momentum acquisition.

Abstract

Dark matter haloes in cosmological filaments and walls have their spin vector aligned (in average) with their host structure. While haloes in walls are aligned with the plane of the wall independently of their mass, haloes in filaments present a mass dependent two-regime orientation. Here we show that the transition mass determining the change in the alignment regime (from parallel to perpendicular) depends on the hierarchical level in which the halo is located, reflecting the hierarchical nature of the Cosmic Web. By explicitly exposing this hierarchy we are able to identify the contributions of different components of the filament network to the spin alignment signal. We discuss a unifying picture to describe the alignment of haloes in filaments and walls consistent with previous results and our findings based on a two-phase angular momentum acquisition, first via tidal torquening and later via anisotropic mass accretion. The hierarchical identification and characterization of cosmic structures was done with a new implementation of the Multiscale Morphology Filter. The MMF-2 (introduced here) represents a significant improvement over its predecessor in terms of robustness and by explicitly describing the hierarchical relations between the elements of the Cosmic Web.