Shell-like structures in our cosmic neighbourhood
  Signatures of the processes in the early Universe are imprinted in the cosmic
web. Some of them may define shell-like structures characterised by typical
scales. We search for shell-like structures in the distribution of nearby rich
clusters of galaxies drawn from the SDSS DR8. We calculate the distance
distributions between rich clusters of galaxies, and groups and clusters of
various richness, look for the maxima in the distance distributions, and select
candidates of shell-like structures. We analyse the space distribution of
groups and clusters forming shell walls. We find six possible candidates of
shell-like structures, in which galaxy clusters have maxima in the distance
distribution to other galaxy groups and clusters at the distance of about 120
Mpc/h. The rich galaxy cluster A1795, the central cluster of the Bootes
supercluster, has the highest maximum in the distance distribution of other
groups and clusters around them at the distance of about 120 Mpc/h among our
rich cluster sample, and another maximum at the distance of about 240 Mpc/h.
The structures of galaxy systems causing the maxima at 120 Mpc/h form an almost
complete shell of galaxy groups, clusters and superclusters. The richest
systems in the nearby universe, the Sloan Great Wall, the Corona Borealis
supercluster and the Ursa Major supercluster are among them. The probability
that we obtain maxima like this from random distributions is lower than 0.001.
Our results confirm that shell-like structures can be found in the distribution
of nearby galaxies and their systems. The radii of the possible shells are
larger than expected for a BAO shell (approximately 109 Mpc/h versus
approximately 120 Mpc/h), and they are determined by very rich galaxy clusters
and superclusters with high density contrast while BAO shells are barely seen
in the galaxy distribution. We discuss possible consequences of these
differences.
