Intrinsic Correlation of Galaxy Shapes: Implications for Weak Lensing Measurements

Weak gravitational lensing is now established as a powerful method to measure mass fluctuations in the universe. It relies on the measurement of small coherent distortions of the images of background galaxies. Even low-level correlations in the intrinsic shapes of galaxies could however produce a significant spurious lensing signal. These correlations are also interesting in their own right, since their detection would constrain models of galaxy formation. Using about $10^5$ haloes found in N-body simulations, we compute the correlation functions of the intrinsic ellipticity of spiral galaxies assuming that the disk is perpendicular to the angular momentum of the dark matter halo. Although the results are not consistent with zero correlation of ellipticities, the correlation in 3D is generally weak, and we conservatively interpret our results as upper limits on the intrinsic correlation expected in projected catalogues. For deep lensing surveys with median redshifts $\sim 1$, we find that intrinsic correlations on angular scales $\theta \la 1'$ could exceed the lensing signal, and could explain excess signals already reported on these scales. On larger scales, we find limits to the intrinsic correlation function at a level $\sim 10^{-5}$, which gives a (model-dependent) range of separations for which the intrinsic signal is about an order of magnitude below that for lensing. Intrinsic correlations are thus negligible on these scales for dedicated weak lensing surveys. For wider but shallower surveys such as APM and SDSS, intrinsic correlations could dominate the lensing signal. We discuss how such surveys could be used to calibrate the importance of this effect, as well as study spin-spin correlations of spiral galaxies.