Spin-induced galaxy alignments and their implications for weak-lensing measurements

Large-scale correlations in the orientations of galaxies can result from al ignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring protogalaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correl ations using the Zeldovich approximation for a realistic distribution of ga laxy shapes. Weak gravitational lensing can also induce ellipticity correla tions, since the images of neighboring galaxies will be distorted coherentl y. On comparing these two effects that induce shape correlations, we find t hat for current weak-lensing surveys with a median redshift of z(m) = 1, th e intrinsic signal is of the order of 1%-10% of the measured signal. Howeve r, for shallower surveys with z(m) less than or equal to 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments ar e not. This difference can be used to disentangle these two sources of elli pticity correlations.