Assembly History and Structure of Galactic Cold Dark Matter Halos

TL;DR

This study uses Aquarius simulations to analyze how the assembly history influences the structure of galaxy-mass dark matter halos, revealing that most mass is accumulated through diffuse accretion and minor mergers, with major mergers having limited impact.

Contribution

It provides new insights into the relative importance of minor mergers and diffuse accretion in halo growth, challenging previous perceptions of hierarchical assembly.

Findings

Major mergers contribute less than 20% to total halo mass.

Most baryonic matter is accreted diffusely rather than through mergers.

A significant fraction of ejected material was previously part of earlier halos.

Abstract

We use the Aquarius simulation series to study the imprint of assembly history on the structure of Galaxy-mass cold dark matter halos. Our results confirm earlier work regarding the influence of mergers on the mass density profile and the inside-out growth of halos. The inner regions that contain the visible galaxies are stable since early times and are significantly affected only by major mergers. Particles accreted diffusely or in minor mergers are found predominantly in the outskirts of halos. Our analysis reveals trends that run counter to current perceptions of hierarchical halo assembly. For example, major mergers (i.e. those with progenitor mass ratios greater than 1:10) contribute little to the total mass growth of a halo, on average less than 20 per cent for our six Aquarius halos. The bulk is contributed roughly equally by minor mergers and by "diffuse" material which is not resolved into individual objects. This is consistent with modeling based on excursion-set theory which suggests that about half of this diffuse material should not be part of a halo of any scale. Interestingly, the simulations themselves suggest that a significantly fraction is not truly diffuse, since it was ejected from earlier halos by mergers prior to their joining the main system. The Aquarius simulations resolve halos to much lower mass scales than are expected to retain gas or form stars. These results thus confirm that most of the baryons from which visible galaxies form are accreted diffusely, rather than through mergers, and they suggest that only relatively rare major mergers will affect galaxy structure at later times.