The Shape of Dark Matter Haloes in the Aquarius Simulations: Evolution and Memory

TL;DR

This study uses high-resolution simulations to analyze how the shapes of Milky Way-like dark matter haloes evolve over time, revealing a transition from prolate to triaxial/oblate configurations influenced by accretion history.

Contribution

It provides detailed insights into the time-dependent shape evolution of dark matter haloes and links shape changes to accretion patterns and assembly history.

Findings

Halo shapes evolve from prolate to triaxial/oblate over time.

Inner and outer halo shapes record different assembly histories.

Shape variations at different radii reflect past accretion conditions.

Abstract

We use the high resolution cosmological N-body simulations from the Aquarius project to investigate in detail the mechanisms that determine the shape of Milky Way-type dark matter haloes. We find that, when measured at the instantaneous virial radius, the shape of individual haloes changes with time, evolving from a typically prolate configuration at early stages to a more triaxial/oblate geometry at the present day. This evolution in halo shape correlates well with the distribution of the infalling material: prolate configurations arise when haloes are fed through narrow filaments, which characterizes the early epochs of halo assembly, whereas triaxial/oblate configurations result as the accretion turns more isotropic at later times. Interestingly, at redshift z=0, clear imprints of the past history of each halo are recorded in their shapes at different radii, which also exhibit a variation from prolate in the inner regions to triaxial/oblate in the outskirts. Provided that the Aquarius haloes are fair representatives of Milky Way-like 10^12 Msun objects, we conclude that the shape of such dark matter haloes is a complex, time-dependent property, with each radial shell retaining memory of the conditions at the time of collapse.