Full-sky ray-tracing simulation of weak lensing using ELUCID
  simulations: exploring galaxy intrinsic alignment and cosmic shear
  correlations
  The intrinsic alignment of galaxies is an important systematic effect in
weak-lensing surveys, which can affect the derived cosmological parameters. One
direct way to distinguish different alignment models and quantify their effects
on the measurement is to produce mocked weak-lensing surveys. In this work, we
use full-sky ray-tracing technique to produce mock images of galaxies from the
ELUCID $N$-body simulation run with the WMAP9 cosmology. In our model we assume
that the shape of central elliptical galaxy follows that of the dark matter
halo, and spiral galaxy follows the halo spin. Using the mocked galaxy images,
a combination of galaxy intrinsic shape and the gravitational shear, we compare
the predicted tomographic shear correlations to the results of KiDS and DLS. It
is found that our predictions stay between the KiDS and DLS results. We rule
out a model in which the satellite galaxies are radially aligned with the
center galaxy, otherwise the shear-correlations on small scales are too high.
Most important, we find that although the intrinsic alignment of spiral
galaxies is very weak, they induce a positive correlation between the
gravitational shear signal and the intrinsic galaxy orientation (GI). This is
because the spiral galaxy is tangentially aligned with the nearby large-scale
overdensity, contrary to the radial alignment of elliptical galaxy. Our results
explain the origin of detected positive GI term from the weak-lensing surveys.
We conclude that in future analysis, the GI model must include the dependence
on galaxy types in more detail.
