Tidal alignments as a contaminant of the galaxy bispectrum
  If the orientations of galaxies are correlated with large-scale structure,
then anisotropic selection effects such as preferential selection of face-on
disc galaxies can contaminate large scale structure observables. Here we
consider the effect on the galaxy bispectrum, which has attracted interest as a
way to break the degeneracy between galaxy bias and the amplitude of matter
fluctuations sigma_8. We consider two models of intrinsic galaxy alignments:
one where the probability distribution for the galaxy's orientation contains a
term linear in the local tidal field, appropriate for elliptical galaxies; and
one with a term quadratic in the local tidal field, which may be applicable to
disc galaxies. We compute the correction to the redshift-space bispectrum in
the quasilinear regime, and then focus on its effects on parameter constraints
from the transverse bispectrum, i.e. using triangles in the plane of the sky.
We show that in the linear alignment model, intrinsic alignments result in an
error in the galaxy bias parameters, but do not affect the inferred value of
sigma_8. In contrast, the quadratic alignment model results in a systematic
error in both the bias parameters and sigma_8. However, the quadratic alignment
effect has a unique configuration dependence that should enable it to be
removed in upcoming surveys.
