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Cosmic shear constrains cosmology by exploiting the apparent alignments ofpairs of galaxies due to gravitational lensing by intervening mass clumps.However galaxies may become (intrinsically) aligned with each other, and withnearby mass clumps, during their formation. This effect needs to bedisentangled from the cosmic shear signal to place constraints on cosmology. Weuse the linear intrinsic alignment model as a base and compare it to analternative model and data. If intrinsic alignments are ignored then the darkenergy equation of state is biased by ~50 per cent. We examine how the numberof tomographic redshift bins affects uncertainties on cosmological parametersand find that when intrinsic alignments are included two or more times as manybins are required to obtain 80 per cent of the available information. Weinvestigate how the degradation in the dark energy figure of merit depends onthe photometric redshift scatter. Previous studies have shown that lensing doesnot place stringent requirements on the photometric redshift uncertainty, solong as the uncertainty is well known. However, if intrinsic alignments areincluded the requirements become a factor of three tighter. These results arequite insensitive to the fraction of catastrophic outliers, assuming that thisfraction is well known. We show the effect of uncertainties in photometricredshift bias and scatter. Finally we quantify how priors on the intrinsicalignment model would improve dark energy constraints.

Dark energy constraints from cosmic shear power spectra: impact of intrinsic alignments on photometric redshift requirements