Weak-lensing surveys and the intrinsic correlation of galaxy ellipticities

We explore the possibility that an intrinsic correlation between galaxy ellipticities arising during the galaxy formation process may account for part of the shear signal recently reported by several groups engaged in weak gravitational lensing surveys. Using high-resolution N-body simulations, we measure the projected ellipticities of dark matter halos and their correlations as a function of pair separation. With this simplifying, but not necessarily realistic, assumption (halo shapes as a proxy for galaxy shapes), we find a positive detection of correlations up to scales of at least 20 h(-1) Mpc (limited by the box size). The signal is not strongly affected by variations in the halo-finding technique, or by the resolution of the simulations (over the range tested). We translate our three-dimensional results into angular measurements of ellipticity correlation functions and shear variance, which can be directly compared to observational results. We also make simulated angular surveys by projecting our simulation boxes onto the plane of the sky and applying a radial selection function. Measurements from these catalogs are consistent with the analytic projection of the statistics. Interestingly, the shear variance we measure is a small, but not entirely negligible, fraction (from similar to 10%-20%, depending on the angular scale) of that seen by the observational groups, and the ellipticity correlation functions approximately mimic the functional form expected to be caused by weak lensing. The amplitude of these projected quantities depends strongly on the width in redshift of the galaxy distribution. If in the future photometric redshifts are used to pick out a screen of background galaxies with a small redshift width, then the intrinsic correlation may become comparable to the weak-lensing signal. Although we are dealing with simulated dark matter halos, we might expect there to be a similar sort of signal when real galaxies are used. This could be checked fruitfully using a nearby sample with known redshifts.