The Shapley super-cluster

New X-ray detections and mass distribution

¹ Dipartimento di Scienze Fisiche, Università degli Studi di Napoli “Federico II”, Via Cinthia 9, Compl. Univ. Monte S. Angelo, 80126 Naples, Italy e-mail: betty@na.infn.it
² MIT Kavli Center for Astrophysics and Space Research, Massachusetts Institute of Technology, 70 Vassar Street, Building 37, Cambridge, MA 02139, USA
³ Astrophysics Research Institute, Liverpool John Moores University, Birkenhead CH41 1LD, UK
⁴ Universität Innsbruck, Institut fuer Astrophysik, Technikerstr. 25, 6020 Innsbruck, Austria e-mail: Sabine.Schindler@uibk.ac.at
⁵ Institut für Astrophysik und Extraterrestrische Forschung (IAEF), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: terben@astro.uni-bonn.de

Received: 21 June 2005
Accepted: 5 August 2005

Abstract

The largest and the deepest super-structure known today is the Shapley super-cluster. This is the sky area with the highest over-density of galaxy clusters and therefore also an ideal region to test the effects of a high density environment on galaxies and on clusters. We performed an X-ray survey of a wide region surrounding the Shapley super-structure. Additionally to previously known super-cluster X-ray members, we identified diffuse X-ray emission from 35 cluster candidates without previous X-ray detection. 21 of them were previously known, optically selected super-cluster members, while the other candidates had not been previously detected in any wavelength range. Optical follow-up observations revealed that at least four of these new candidates also have optical cluster counterparts. The super-cluster shows a slightly flattened and elongated morphology. Clusters outside the central dense core are preferentially located in four perpendicular filaments in a similar way to what is seen in simulations of Large Scale Structure. We measure the cluster number density in the region to be more than one order of magnitude higher than the mean density of rich Abell clusters previously observed at similar Galactic latitudes; this over-density, in the super-cluster outskirts, is mainly due to an excess of low X-ray luminous clusters (with respect to an average population), which leads us to think that the whole region is still accreting low luminosity, small objects from the outskirts. Pushing our total X-ray mass estimate to fainter clusters would drastically increase the total super-cluster mass measure, because of the presence of the rich X-ray low luminosity population.