A look to the inside of haloes: a characterisation of the halo shape as a function of overdensity in the Planck cosmology

TL;DR

This study analyzes the shape and internal structure of dark matter haloes across different masses and overdensities, revealing how their triaxiality and orientation evolve with mass, redshift, and formation history.

Contribution

It provides a detailed characterization of halo shapes at various overdensities and introduces distributions useful as priors for mass reconstruction algorithms.

Findings

Halo triaxiality increases with mass and distance from the center.

Inner halo regions retain memory of formation processes.

Outer regions become rounder due to merging events.

Abstract

In this paper we study the triaxial properties of dark matter haloes of a wide range of masses extracted from a set of cosmological N-body simulations. We measure the shape at different distances from the halo centre (characterised by different overdensity thresholds), both in three and in two dimensions. We discuss how halo triaxiality increases with mass, redshift and distance from the halo centre. We also examine how the orientation of the different ellipsoids are aligned with each other and what is the gradient in internal shapes for halos with different virial configurations. Our findings highlight that the internal part of the halo retains memory of the violent formation process keeping the major axis oriented toward the preferential direction of the in-falling material while the outer part becomes rounder due to continuous isotropic merging events. This effect is clearly evident in high mass haloes - which formed more recently - while it is more blurred in low mass haloes. We present simple distributions that may be used as priors for various mass reconstruction algorithms, operating in different wavelengths, in order to recover a more complex and realistic dark matter distribution of isolated and relaxed systems.