Correlations between intrinsic shear and the density field on large scales , a potentially important contaminant for cosmic shear surveys , have been robustly detected at low redshifts with bright galaxies in SDSS data .
Here we present a more detailed characterization of this effect , which can cause anti-correlations between gravitational lensing shear and intrinsic ellipticity ( GI correlations ) .
This measurement uses 36 278 Luminous Red Galaxies ( LRGs ) from the Sloan Digital Sky Survey ( SDSS ) spectroscopic sample with 0.15 < z < 0.35 , split by redshift and luminosity ; 7 758 LRGs from the 2dF-SDSS LRG and QSO ( 2SLAQ ) Survey at 0.4 < z < 0.8 ; and a variety of other SDSS samples from previous , related work .
We find > 3 \sigma detections of the effect on large scales ( up to 60 h ^ { -1 } Mpc ) for all galaxy subsamples within the SDSS LRG sample ; for the 2SLAQ sample , we find a 2 \sigma detection for a bright subsample , and no detection for a fainter subsample .
Fitting formulae are provided for the scaling of the GI correlations with luminosity , transverse separation , and redshift ( for which the 2SLAQ sample , while small , provides crucial constraints due to its longer baseline in redshift ) .
We estimate contamination in the measurement of \sigma _ { 8 } for future cosmic shear surveys on the basis of the fitted dependence of GI correlations on galaxy properties .
We find contamination to the power spectrum ranging from -1.5 ( optimistic ) to -33 per cent ( pessimistic ) for a toy cosmic shear survey using all galaxies to a depth of R = 24 using scales l \approx 500 , though the central value of predicted contamination is -6.5 per cent .
This corresponds to a bias in \sigma _ { 8 } of \Delta \sigma _ { 8 } = -0.004 ( optimistic ) , -0.02 ( central ) , or -0.10 ( pessimistic ) .
We provide a prescription for inclusion of this error in cosmological parameter estimation codes .
The principal uncertainty is in the treatment of the L \leq L _ { \star } blue galaxies , for which we have no detection of the GI signal , but which could dominate the GI contamination if their GI amplitude is near our upper limits .
Characterization of the tidal alignments of these galaxies , especially at redshifts relevant for cosmic shear , should be a high priority for the cosmic shear community .