Angular ellipticity correlations in a composite alignment model for
  elliptical and spiral galaxies and inference from weak lensing
  We investigate a physical, composite alignment model for both spiral and
elliptical galaxies and its impact on cosmological parameter estimation from
weak lensing for a tomographic survey. Ellipticity correlation functions and
angular ellipticity spectra for spiral and elliptical galaxies are derived on
the basis of tidal interactions with the cosmic large-scale structure and
compared to the tomographic weak lensing signal. We find that elliptical
galaxies cause a contribution to the weak-lensing dominated ellipticity
correlation on intermediate angular scales between $\ell\simeq40$ and
$\ell\simeq400$ before that of spiral galaxies dominates on higher multipoles.
The predominant term on intermediate scales is the negative cross-correlation
between intrinsic alignments and weak gravitational lensing (GI-alignment). We
simulate parameter inference from weak gravitational lensing with intrinsic
alignments unaccounted; the bias induced by ignoring intrinsic alignments in a
survey like Euclid is shown to be several times larger than the statistical
error and can lead to faulty conclusions when comparing to other observations.
The biases generally point into different directions in parameter space, such
that in some cases one can observe a partial cancellation effect. Furthermore,
it is shown that the biases increase with the number of tomographic bins used
for the parameter estimation process. We quantify this parameter estimation
bias in units of the statistical error and compute the loss of Bayesian
evidence for a model due to the presence of systematic errors as well as the
Kullback-Leibler divergence to quantify the distance between the true model and
the wrongly inferred one.
