Intrinsic alignments of galaxies in the MassiveBlack-II simulation:
  analysis of two-point statistics
  The intrinsic alignment of galaxies with the large-scale density field is an
important astrophysical contaminant in upcoming weak lensing surveys. We
present detailed measurements of the galaxy intrinsic alignments and associated
ellipticity-direction (ED) and projected shape ($w_{g+}$) correlation functions
for galaxies in the cosmological hydrodynamic MassiveBlack-II (MB-II)
simulation. We carefully assess the effects on galaxy shapes, misalignment of
the stellar component with the dark matter shape and two-point statistics of
iterative weighted (by mass and luminosity) definitions of the (reduced and
unreduced) inertia tensor. We find that iterative procedures must be adopted
for a reliable measurement of the reduced tensor but that luminosity versus
mass weighting has only negligible effects. Both ED and $w_{g+}$ correlations
increase in amplitude with subhalo mass (in the range of $10^{10} - 6.0\times
10^{14}h^{-1}M_{\odot}$), with a weak redshift dependence (from $z=1$ to
$z=0.06$) at fixed mass. At $z \sim 0.3$, we predict a $w_{g+}$ that is in
reasonable agreement with SDSS LRG measurements and that decreases in amplitude
by a factor of $\sim 5$--18 for galaxies in the LSST survey. We also compared
the intrinsic alignments of centrals and satellites, with clear detection of
satellite radial alignments within their host halos. Finally, we show that
$w_{g+}$ (using subhalos as tracers of density) and $w_{\delta+}$ (using dark
matter density) predictions from the simulations agree with that of non-linear
alignment models (NLA) at scales where the 2-halo term dominates in the
correlations (and tabulate associated NLA fitting parameters). The 1-halo term
induces a scale dependent bias at small scales which is not modeled in the NLA
model.
