Intrinsic Alignments of Galaxies in the Illustris Simulation
  We study intrinsic alignments (IA) of galaxy image shapes within the
Illustris cosmic structure formation simulations. We investigate how IA
correlations depend on observable galaxy properties such as stellar mass,
apparent magnitude, redshift, and photometric type, and on the employed shape
measurement method. The correlations considered include the matter
density-intrinsic ellipticity (mI), galaxy density-intrinsic ellipticity (dI),
gravitational shear-intrinsic ellipticity (GI), and intrinsic
ellipticity-intrinsic ellipticity (II) correlations. We find stronger
correlations for more massive and more luminous galaxies, as well as for
earlier photometric types, in agreement with observations. Moreover, the
correlations significantly depend on the choice of shape estimator, even if
calibrated to serve as unbiased shear estimators. In particular, shape
estimators that down-weight the outer parts of galaxy images produce much
weaker IA signals on intermediate and large scales than methods employing flat
radial weights. The expected contribution of intrinsic alignments to the
observed ellipticity correlation in tomographic cosmic shear surveys may be
below one percent or several percent of the full signal depending on the
details of the shape measurement method. A comparison of our results to a tidal
alignment model indicates that such a model is able to reproduce the IA
correlations well on intermediate and large scales, provided the effect of
varying galaxy density is correctly taken into account. We also find that the
GI contributions to the observed ellipticity correlations could be inferred
directly from measurements of galaxy density-intrinsic ellipticity
correlations, except on small scales, where systematic differences between mI
and dI correlations are large.
