Detection of galaxy spin alignments in the point source catalog redshift survey shear field

We report the first direct observational evidence for the alignment of galaxy spin axes with the local tidal shear field. We measure quantitatively the strength of this directional correlation of disk galaxies from the Tully catalog, with the local shear field reconstructed from Point Source Catalog Redshift survey galaxies. We demonstrate that the null hypothesis of random galaxy alignments relative to the shear frame can be ruled out with more than 99.98% confidence. The observed intrinsic correlation averaged over the censored samples that have detected nonzero signals is measured in terms of the alignment parameter (a) over bar = 0.17+/-0.04, which includes only statistical errors of the censored data but not the cosmic variance error. The reconstruction procedure is likely to underestimate (a) over bar systematically. Our result is consistent with the linear tidal-torque model and supports the idea that the present galaxy spins may be used as a probe of primordial tidal shear and mass density fields. The intrinsic alignments of galaxy spins dominate over weak gravitational lensing for shallow surveys such as the Sloan Digital Sky Survey, while it should be negligible for deeper surveys at z similar to 1.