An analytical framework to describe the orientation of dark matter halos and galaxies within the large-scale structure

TL;DR

This paper develops a comprehensive analytical framework to study the orientations and alignments of dark matter halos and galaxies within large-scale cosmic structures, incorporating models, statistical tools, and observational considerations.

Contribution

It introduces new models and statistical methods to describe halo and galaxy alignments with large-scale structures, validated against numerical simulations and observational data.

Findings

Alignment strength in voids is reduced to half due to halo angular momentum rotation.

The proposed models accurately recover simulation results for halo orientations.

The framework can be adapted to various cosmic structures like filaments and walls.

Abstract

We provide a set of general tools for studying the alignments of dark matter halos and galaxies with respect to the large scale structure. The statistics of the positioning of these objects is represented by a Probability Distribution Function (PDF) of their Euler angles. The PDF corresponding to halos located in the shells of the cosmic voids is inferred from previous results. This PDF is used to show how to recover the outcomes found for the alignments of the axes of these halos in simulations. We also explore the orientation of the angular momentum of the halos, both with respect to the halo axes and with respect to the large scale structure. We present an expression which describes well numerical results for the alignment of the angular momentum of the halo with respect to the halo axes for randomly chosen halos. We also propose a model that relates the orientation of the angular momentum with the halos axes accounting for the orientation of the halo axes with the large scale structure. This model is shown to recover accurately the observed PDF of the halo angular momentum with respect to the void radial direction. In addition, we give an expression for determining the degradation of the angular momentum intrinsic alignment when observational errors are accounted. This expression is also used to determine the departure of the observed value of the alignment from the initial expectation (as provided by the tidal torque theory) due to the rotation of the angular momentum of the halo with respect to the initial torque. For voids, we find that the strength of the alignment is reduced to half the original value. We discuss how to adapt the void results to other cosmic large scale structures (i.e. filaments, walls, etc).