Density profile of dark matter haloes and galaxies in the Horizon-AGN
  simulation: the impact of AGN feedback
  Using a suite of three large cosmological hydrodynamical simulations,
Horizon-AGN, Horizon-noAGN (no AGN feedback) and Horizon-DM (no baryons), we
investigate how a typical sub-grid model for AGN feedback affects the evolution
of the inner density profiles of massive dark matter haloes and galaxies. Based
on direct object-to-object comparisons, we find that the integrated inner mass
and density slope differences between objects formed in these three simulations
(hereafter, H_AGN, H_noAGN and H_DM) significantly evolve with time. More
specifically, at high redshift (z~5), the mean central density profiles of
H_AGN and H_noAGN dark matter haloes tend to be much steeper than their H_DM
counterparts owing to the rapidly growing baryonic component and ensuing
adiabatic contraction. By z~1.5, these mean halo density profiles in H_AGN have
flattened, pummelled by powerful AGN activity ("quasar mode"): the integrated
inner mass difference gaps with H_noAGN haloes have widened, and those with
H_DM haloes have narrowed. Fast forward 9.5 billion years, down to z=0, and the
trend reverses: H_AGN halo mean density profiles drift back to a more cusped
shape as AGN feedback efficiency dwindles ("radio mode"), and the gaps in
integrated central mass difference with H_noAGN and H_DM close and broaden
respectively. On the galaxy side, the story differs noticeably. Averaged
stellar profile central densities and inner slopes are monotonically reduced by
AGN activity as a function of cosmic time, resulting in better agreement with
local observations. As both dark matter and stellar inner density profiles
respond quite sensitively to the presence of a central AGN, there is hope that
future observational determinations of these quantities can be used constrain
AGN feedback models.
