Influence of Orbital Behaviour and Chaos on the Shape of Dark Matter Halos

TL;DR

This study investigates how baryonic matter influences dark matter halo shapes by analyzing particle orbits, revealing that baryonic growth induces chaos and results in rounder halo configurations.

Contribution

First detailed analysis of particle orbits in cosmological dark matter halos showing baryonic growth causes orbit chaos and shape transformation.

Findings

Baryonic infall deflects orbits visiting the halo center.

Growth of baryonic mass induces orbit chaos and phase space exploration.

Halo becomes rounder due to orbit randomization caused by baryonic effects.

Abstract

It has been shown that the dissipative gas infall during galaxy formation has the capability to modify the shape of dark matter halos. In this paper we perform the first detailed analysis of particle orbits in a cosmological dark matter halos to understand {\it how} and {\it why} baryons alter its shape. We perform a series of numerical experiments where we grow a baryonic core inside a live dark matter halo extracted from a cosmological simulation. We follow the orbits of more than 300 particles with more that 50000 timesteps. Our results clearly show that the dissipational component is responsible for repeatedly deflecting orbits which visit often the center of the system. Moreover the gravitational potential time dependence associated with the growth of the baryonic mass, shifts the frequencies of the orbits, making them extremely chaotic. This randomization makes the orbits explore a large phase space. When this effect takes place for a significant number of orbits it will be manifested in the density distribution as an approach to a rounder configuration. As a consequence, the influence of the central mass on the shape of the phase space decreases with increasing distance from the center. We discuss the importance of future analysis of controlled experiments (i.e. using analytic potentials instead of live DM halos) to better decipher the dynamics of this phenomenon.