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).
