The Cosmological Impact of Intrinsic Alignment Model Choice for Cosmic
  Shear
  We consider the effect of galaxy intrinsic alignments (IAs) on dark energy
constraints from weak gravitational lensing. We summarise the latest version of
the linear alignment model of IAs, following the brief note of Hirata & Seljak
(2010) and further interpretation in Laszlo et al. (2011). We show the
cosmological bias on the dark energy equation of state parameters w0 and wa
that would occur if IAs were ignored. We find that w0 and wa are both
catastrophically biased, by an absolute value of just greater than unity under
the Fisher matrix approximation. This contrasts with a bias several times
larger for the earlier IA implementation. Therefore there is no doubt that IAs
must be taken into account for future Stage III experiments and beyond. We use
a flexible grid of IA and galaxy bias parameters as used in previous work, and
investigate what would happen if the universe used the latest IA model, but we
assumed the earlier version. We find that despite the large difference between
the two IA models, the grid flexibility is sufficient to remove cosmological
bias and recover the correct dark energy equation of state. In an appendix, we
compare observed shear power spectra to those from a popular previous
implementation and explain the differences.
