  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 36278 Luminous Red Galaxies (LRGs)
from the SDSS spectroscopic sample with 0.15<z<0.35, split by redshift and
luminosity; 7758 LRGs from the 2SLAQ Survey at 0.4<z<0.8; and a variety of
other SDSS samples from previous, related work. We find >3sigma detections of
the effect for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ
sample, we find a 2sigma 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. 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~500. 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<=L*
blue galaxies. 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. (Abridged)
