Some like it triaxial: the universality of dark matter halo shapes and their evolution along the cosmic time

TL;DR

This study analyzes dark matter halo shapes across different cosmological simulations, revealing universal relations between halo shape parameters and mass that are consistent over cosmic time and various cosmological models.

Contribution

It introduces a rescaling method using the variable ν to establish universal shape-mass relations for dark matter halos across different redshifts and cosmologies.

Findings

Halos tend to be prolate at all times.

More massive halos are less spherical.

Universal shape-mass relations are found using the variable ν.

Abstract

We present a detailed analysis of dark matter halo shapes, studying how the distributions of ellipticity, prolateness and axial ratios evolve as a function of time and mass. With this purpose in mind, we analysed the results of three cosmological simulations, running an ellipsoidal halo finder to measure triaxial halo shapes. The simulations have different scales, mass limits and cosmological parameters, which allows us to ensure a good resolution and statistics in a wide mass range, and to investigate the dependence of halo properties on the cosmological model. We confirm the tendency of haloes to be prolate at all times, even if they become more triaxial going to higher redshifts. Regarding the dependence on mass, more massive haloes are also less spherical at all redshifts, since they are the most recent forming systems and so still retain memory of their original shape at the moment of collapse. We then propose a rescaling of the shape-mass relations, using the variable $\nu = \delta_{c}/\sigma$ to represent the mass, which absorbs the dependence on both cosmology and time, allowing to find universal relations between halo masses and shape parameters (ellipticity, prolateness and the axial ratios) which hold at any redshift. This may be very useful to determine prior distributions of halo shapes for observational studies.