Galaxy Alignments in Very X-ray Luminous Clusters at z>0.5
  We present the results of a search for galaxy alignments in 12 galaxy
clusters at z>0.5, a statistically complete subset of the very X-ray luminous
clusters from the MAssive Cluster Survey (MACS). Using high-quality images
taken with the Hubble Space Telescope (HST) that render measurement errors
negligible, we find no radial galaxy alignments within 500 kpc of the cluster
centres for a sample of 545 spectroscopically confirmed cluster members. A
mild, but statistically insignificant trend favouring radial alignments is
observed within a radius of 200 kpc and traced to galaxies on the cluster red
sequence. Our results for massive clusters at z>0.5 are in stark contrast to
the findings of previous studies which find highly significant radial
alignments of galaxies in nearby clusters at z~0.1 out to at least half the
virial radius using imaging data from the SDSS. The discrepancy becomes even
more startling if radial alignment becomes more prevalent at decreasing
clustercentric distance, as suggested by both our and previous work. We
investigate and discuss potential causes for the disparity between our findings
based on HST images of clusters at z>0.5 and those obtained using groundbased
images of systems at z~0.1. We conclude that the most likely explanation is
either dramatic evolution with redshift (in the sense that radial alignments
are less pronounced in dynamically younger systems) or the presence of
systematic biases in the analysis of SDSS imaging data that cause at least
partly spurious alignment signals.
