Constraints on the Alignment of Galaxies in Galaxy Clusters from
  $\sim$14,000 Spectroscopic Members
  Torques acting on galaxies lead to physical alignments, but the resulting
ellipticity correlations are difficult to predict. As they constitute a major
contaminant for cosmic shear studies, it is important to constrain the
intrinsic alignment signal observationally. We measured the alignments of
satellite galaxies within 90 massive galaxy clusters in the redshift range
0.05<z<0.55 and quantified their impact on the cosmic shear signal. We combined
a sample of 38,104 galaxies with spectroscopic redshifts with high-quality data
from the Canada-France-Hawaii Telescope. We used phase-space information to
select 14,576 cluster members, 14,250 of which have shape measurements and
measured three different types of alignment: the radial alignment of satellite
galaxies toward the brightest cluster galaxies (BCGs), the common orientations
of satellite galaxies and BCGs, and the radial alignments of satellites with
each other. Residual systematic effects are much smaller than the statistical
uncertainties. We detect no galaxy alignment of any kind out to at least 3
r200. The signal is consistent with zero for both blue and red galaxies, bright
and faint ones, and also for subsamples of clusters based on redshift,
dynamical mass, and dynamical state. These conclusions are unchanged if we
expand the sample with bright cluster members from the red sequence. We augment
our constraints with those from the literature to estimate the importance of
the intrinsic alignments of satellites compared to those of central galaxies,
for which the alignments are described by the linear alignment model.
Comparison of the alignment signals to the expected uncertainties of current
surveys such as the Kilo-Degree Survey suggests that the linear alignment model
is an adequate treatment of intrinsic alignments, but it is not clear whether
this will be the case for larger surveys.
