Intrinsic correlation of halo ellipticity and its implications for   large-scale weak lensing surveys
  We use a large set of state-of-the-art cosmological N-body simulations [512^3 particles] to study the intrinsic ellipticity correlation functions of halos. With the simulations of different resolutions, we find that the ellipticity correlations converge once the halos have more than 160 members. For halos with fewer members, the correlations are underestimated, and the underestimation amounts to a factor of 2 when the halos have only 20 particles. After correcting for the resolution effects, we show that the ellipticity correlations of halos in the bigger box (L=300 mpc) agree very well with those obtained in the smaller box (L=100 mpc). Combining these results from the different simulation boxes, we present accurate fitting formulae for the ellipticity correlation function c_{11}(r) and for the projected correlation functions Sigma_{11}(r_p) and Sigma_{22}(r_p) over three orders of magnitude in halo mass. The latter two functions are useful for predicting the contribution of the intrinsic correlations to deep lensing surveys. With reasonable assumptions for the redshift distribution of galaxies and for the mass of galaxies, we find that the intrinsic ellipticity correlation can contribute significantly not only to shallow surveys but also to deep surveys. Our results indicate that previous similar studies significantly underestimated this contribution for their limited simulation resolutions. 
Y.P. Jing (Partner Group of MPA, Shanghai Observatory)