Baryon-driven decontraction in Milky Way mass haloes
  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.
