  Cosmic shear constrains cosmology by exploiting the apparent alignments of
pairs of galaxies due to gravitational lensing by intervening mass clumps.
However galaxies may become (intrinsically) aligned with each other, and with
nearby mass clumps, during their formation. This effect needs to be
disentangled from the cosmic shear signal to place constraints on cosmology. We
use the linear intrinsic alignment model as a base and compare it to an
alternative model and data. If intrinsic alignments are ignored then the dark
energy equation of state is biased by ~50 per cent. We examine how the number
of tomographic redshift bins affects uncertainties on cosmological parameters
and find that when intrinsic alignments are included two or more times as many
bins are required to obtain 80 per cent of the available information. We
investigate how the degradation in the dark energy figure of merit depends on
the photometric redshift scatter. Previous studies have shown that lensing does
not place stringent requirements on the photometric redshift uncertainty, so
long as the uncertainty is well known. However, if intrinsic alignments are
included the requirements become a factor of three tighter. These results are
quite insensitive to the fraction of catastrophic outliers, assuming that this
fraction is well known. We show the effect of uncertainties in photometric
redshift bias and scatter. Finally we quantify how priors on the intrinsic
alignment model would improve dark energy constraints.
