  Correlations between intrinsic ellipticities of galaxies are a potentially
important systematic error when constraining dark energy properties from weak
gravitational lensing (cosmic shear) surveys. In the absence of perfectly known
galaxy redshifts some modeling of the galaxy intrinsic alignments is likely to
be required to extract the lensing signal to sufficient accuracy. We present a
new model based on the placement of galaxies into dark matter halos. The
central galaxy ellipticity follows the large scale potential and, in the
simplest case, the satellite galaxies point at the halo center. The two-halo
term is then dominated by the linear alignment model and the one-halo term
provides a motivated extension of intrinsic alignment models to small scales.
We provide fitting formulae for the spatial projected source power spectra for
both intrinsic-intrinsic (II) and shear-intrinsic (GI) correlations. We
illustrate the potential impact of ignoring intrinsic alignments on
cosmological parameter constraints from non-tomographic surveys, finding that
sigma_8 could be underestimated by up to the size of the current 1-sigma error
bar from cosmic shear if very small scales are included in the analysis.
Finally, we highlight areas of interest for numerical simulations of dark
matter clustering and galaxy formation that can further constrain the intrinsic
alignment signal.
