Baryon-driven decontraction in Milky Way mass haloes

TL;DR

This study uses high-resolution simulations to explore how baryonic processes like AGN feedback and stellar bars can cause decontraction of dark matter haloes in Milky Way-like galaxies, affecting mass estimates and detection signals.

Contribution

It identifies two baryonic mechanisms—AGN-driven gas blowouts and stellar bar interactions—that can reduce the central dark matter mass in galaxy haloes, challenging traditional contraction models.

Findings

AGN feedback can significantly decrease central dark matter mass.

Stellar bars induce secular halo expansion depending on their strength.

Inner halo mass at present is less than in the past and less than adiabatic contraction predictions.

Abstract

We select a sample of Milky Way (MW) mass haloes from a high-resolution version of the EAGLE simulation to study their inner dark matter (DM) content and how baryons alter it. As in previous studies, we find that all haloes are more massive at the centre compared to their DM-only (DMO) counterparts at the present day as a result of the dissipational collapse of baryons during the assembly of the galaxy. However, we identify two processes that can reduce the central halo mass during the evolution of the galaxy. Firstly, gas blowouts induced by AGN feedback can lead to a substantial decrease of the central DM mass. Secondly, the formation of a stellar bar and its interaction with the DM can induce a secular expansion of the halo; the rate at which DM is evacuated from the central region by this process is related to the average bar strength and the timescale on which it acts determines how much the halo has decontracted. Although the inner regions of the haloes we have investigated are still more massive than their DMO counterparts at $z = 0$, they are significantly less massive than in the past and less massive than expected from the classic adiabatic contraction model. Since the MW has both a central supermassive black hole and a bar, the extent to which its halo has contracted is uncertain. This may affect estimates of the mass of the MW halo and of the expected signals in direct and indirect DM detection experiments.