Intrinsic correlation of galaxy shapes: implications for weak lensing measurements

Weak gravitational lensing is now established as a powerful method to measu re 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 si gnificant spurious lensing signal. These correlations are also interesting in their own right, since their detection would constrain models of galaxy formation. Using 3 x 10(4)-10(5) haloes found in N-body simulations, we com pute the correlation functions of the intrinsic ellipticity of spiral galax ies assuming that the disc is perpendicular to the angular momentum of the dark matter halo. We also consider a simple model for elliptical galaxies, in which the shape of the dark matter halo is assumed to be the same as tha t of the light. For deep lensing surveys with median redshifts similar to1, we find that intrinsic correlations of similar to 10(-4) on angular scales theta similar to 0.1-10 arcmin are generally below the expected lensing si gnal, and contribute only a small fraction of the excess signals reported o n these scales. On larger scales we find limits to the intrinsic correlatio n function at a level similar to 10(-5), which gives a (model-dependent) ra nge of separations for which the intrinsic signal is about an order of magn itude below the ellipticity correlation function expected from weak lensing . Intrinsic correlations are thus negligible on these scales for dedicated weak lensing surveys. For wider but shallower surveys such as SuperCOSMOS, APM and SDSS, we cannot exclude the possibility chat intrinsic correlations could dominate the lensing signal. We discuss how such surveys could be us ed to calibrate the importance of this effect, as well as study spin-spin c orrelations of spiral galaxies.