Shell-like structures in our cosmic neighbourhood

¹ Tartu Observatory, 61602 Tõravere, Estonia
e-mail: maret.einasto@to.ee
² Estonian Academy of Sciences, 10130 Tallinn, Estonia
³ Institute of Physics, Tartu University, Tähe 4, 51010 Tartu, Estonia
⁴ Observatori Astronòmic, Universitat de València, c/ Catedràtic Josè Beltràn, 2, 46980 Paterna, València, Spain
⁵ ICRANet, Piazza della Repubblica 10, 65122 Pescara, Italy
⁶ Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 17 June 2015
Accepted: 8 January 2016

Abstract

Context. 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. Examples of such structures are shell-like systems of galaxies, which are interpreted as a signatures of the baryon acoustic oscillations.

Aims. We search for shell-like structures in the distribution of nearby rich clusters of galaxies drawn from the SDSS DR8.

Methods. We calculated the distance distributions between rich clusters of galaxies and groups and clusters of various richness, searched for the maxima in the distance distributions and selected candidates of shell-like structures. We analysed the space distribution of groups and clusters that form shell walls.

Results. We find six possible candidates of shell-like structures, in which galaxy clusters have the maximum in their distance distribution to other galaxy groups and clusters at a distance of about 120−130 h^-1 Mpc. Another, less probable maximum is found at a distance of about 240 h^-1 Mpc. The rich galaxy cluster A1795, which is the central cluster of the Bootes supercluster, has the highest maximum in the distance distribution of all other surrounding groups and clusters in our rich cluster sample. It lies at a distance of about 120 h^-1 Mpc. The structures of galaxy systems that cause this maximum 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.

Conclusions. 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 baryonic acoustic oscillations (BAO) shell (≈109 h^-1 Mpc versus ≈120–130 h^-1 Mpc), and they are determined by very rich galaxy clusters and superclusters. In contrast, BAO shells are barely seen in the galaxy distribution. We discuss possible consequences of these differences.