Detection of intrinsic cluster alignments to 100 Mpc/h in the SDSS
  We measure the large-scale intrinsic alignments of galaxy clusters in the
Sloan Digital Sky Survey (SDSS) using subsets of two cluster catalogues: 6625
clusters with 0.1<z<0.3 from the maxBCG cluster catalogue (Koester et al. 2007,
7500 sq. deg.), and 8081 clusters with 0.08<z<0.44 from the Adaptive Matched
Filter catalogue (Dong et al. 2008, 6500 sq. deg.). We search for two types of
cluster alignments using pairs of clusters: the alignment between the projected
major axes of the clusters (`correlation' alignment), and the alignment between
one cluster major axis and the line connecting it to the other cluster in the
pair (`pointing' alignment). In each case, we use the cluster member galaxy
distribution as a tracer of the cluster shape. All measurements are carried out
with each catalogue separately, to check for dependence on cluster selection
procedure. We find a strong detection of the pointing alignment on scales up to
100 Mpc/h, at the 6 or 10-sigma level depending on the cluster selection
algorithm used. The correlation alignment is only marginally detected up to ~20
Mpc/h, at the 2 or 2.5-sigma level. These results support our current
theoretical understanding of galaxy cluster intrinsic alignments in the LCDM
paradigm, although further work will be needed to understand the impact of
cluster selection effects and observational measurement errors on the amplitude
of the detection.
