A Radial Measurement of the Galaxy Tidal Alignment Magnitude with BOSS Data

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 Hirata 2009, 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 $0.51\pm 0.32$ ($0.77\pm0.41$) for CMASS (LOWZ). We combine the results to obtain a total ${\rm {Obs}/{Theory}} = 0.61\pm 0.26$. This measurement constitutes evidence (between 2 and 3$\sigma$) for radial intrinsic alignments, and is consistent with theoretical expectations ($<2\sigma$ difference).