On the Edge: the relation between stellar and dark matter haloes of Milky Way-mass galaxies

TL;DR

This study uses cosmological simulations to explore how stellar and dark matter haloes of Milky Way-like galaxies form and relate, revealing that stellar halos mainly originate from disrupted massive dwarfs, while dark matter haloes are dominated by unresolved subhaloes and smooth components.

Contribution

It provides new insights into the connection between stellar and dark matter haloes, especially how features like the halo edge relate to dark matter halo properties and assembly history.

Findings

Stellar halos are primarily formed from stars stripped from a few massive dwarfs.

Dark matter halos mainly consist of unresolved subhaloes and a smooth component.

The stellar halo edge can be used to infer dark matter halo mass and assembly history.

Abstract

We investigate the build-up of the accreted stellar and dark matter haloes of Milky Way-like galaxies in APOSTLE suite of cosmological hydrodynamics simulations. We show that the stellar halo is made up primarily of stars stripped from a small number of massive dwarfs, most of which are disrupted by the present day. The dark matter halo, on the other hand, is made up primarily of small unresolved subhaloes ($\lesssim 10^6$ M$_{\odot}$) and a ``smooth'' component consisting of particles which were never bound to a subhalo. Despite these differences, the massive dwarfs that make up the majority of the stellar halo also contribute a significant fraction of the dark matter. The stars and dark matter stripped from these dwarfs are related through their kinematics and this leaves imprints in the phase-space structure of the haloes. We examine the relation between the location of features, such as caustics, in the phase space of the stars and dark halo properties. We show that the ``edge'' of the stellar halo is a probe of dark matter halo mass and assembly history. The edges of Milky Way-mass galaxies should be visible at a surface brightness of 31-36 mag arcsec$^{-2}$.