The anisotropy of galaxy clustering in redshift space has long been used to probe the rate of growth of cosmological perturbations .
However , if galaxies are aligned by large-scale tidal fields , then a sample with an orientation-dependent selection effect has an additional anisotropy imprinted onto its correlation function .
We use the LOWZ and CMASS catalogs of SDSS-III BOSS Data Release 12 to divide galaxies into two sub-samples based on their offset from the Fundamental Plane , which should be correlated with orientation .
These sub-samples must trace the same underlying cosmology , but have opposite orientation-dependent selection effects .
We measure the clustering parameters of each sub-sample and compare them in order to calculate the dimensionless parameter B , a measure of how strongly galaxies are aligned by gravitational tidal fields .
We found that for CMASS ( LOWZ ) , the measured B was -0.024 \pm 0.015 ( -0.030 \pm 0.016 ) .
This result can be compared to the theoretical predictions of [ ] , who argued that since galaxy formation physics does not depend on the direction of the ‘ ‘ observer , ’ ’ the same intrinsic alignment parameters that describe galaxy-ellipticity correlations should also describe intrinsic alignments in the radial direction .
We find that the ratio of observed to theoretical values is 0.51 \pm 0.32 ( 0.77 \pm 0.41 ) for CMASS ( LOWZ ) .
We combine the results to obtain a total { Obs / { Theory } } = 0.61 \pm 0.26 .
This measurement constitutes evidence ( between 2 and 3 \sigma ) for radial intrinsic alignments , and is consistent with theoretical expectations ( < 2 \sigma difference ) .