Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations
  We report results for the alignments of galaxies in the EAGLE and cosmo-OWLS
simulations as a function of galaxy separation and halo mass. The combination
of these hydro-cosmological simulations enables us to span four orders of
magnitude in halo mass ($10.7<log_{10}(M_{200}/[h^{-1}M_\odot])<15$) and a
large range of separations ($-1<log_{10}(r/[h^{-1}Mpc])< 2$). We focus on two
classes of alignments: the orientations of galaxies with respect to either the
directions to, or the orientations of, surrounding galaxies. We find that the
strength of the alignment is a strongly decreasing function of the distance
between galaxies. The orientation-direction alignment can remain significant up
to ~100 Mpc, for galaxies hosted by the most massive haloes in our simulations.
Galaxies hosted by more massive subhaloes show stronger alignment. At a fixed
halo mass, more aspherical or prolate galaxies exhibit stronger alignments. The
spatial distribution of satellites is anisotropic and significantly aligned
with the major axis of the main host halo. The major axis of satellite
galaxies, when all stars are considered, are preferentially aligned towards the
centre of the main host halo. The predicted projected direction-orientation
alignment, $\epsilon_{g+}(r_{p})$, is in broad agreement with recent
observations when only stars within the typical observable extent of a galaxy
are used to define galaxy orientations. We find that the
orientation-orientation alignment is weaker than the orientation-direction
alignment on all scales. Overall, the strength of galaxy alignments depends
strongly on the subset of stars that are used to measure the orientations of
galaxies and it is always weaker than the alignment of the dark matter haloes.
Thus, alignment models that use halo orientation as a direct proxy for galaxy
orientation will overestimate the impact of intrinsic alignments on weak
lensing analyses.
