The XXL Survey VII: A supercluster of galaxies at z=0.43

TL;DR

This paper reports the discovery of the highest redshift supercluster of galaxies at z=0.43 in the XXL Survey, revealing a compact, massive structure with unique features that enhances understanding of large-scale cosmic formation.

Contribution

It presents the first supercluster discovered with the XXL Survey, identified through a new application of a friends-of-friends algorithm to X-ray data, and provides detailed properties and optical follow-up.

Findings

Discovered a supercluster at z=0.43 with six galaxy clusters.

The supercluster is very compact, spanning 10 by 5 Mpc.

It has an average mass range for superclusters and an unusual appearance.

Abstract

The XXL Survey is the largest homogeneous and contiguous survey carried out with XMM-Newton. Covering an area of 50 square degrees distributed over two fields, it primarily investigates the large-scale structures of the Universe using the distribution of galaxy clusters and active galactic nuclei as tracers of the matter distribution. Given its depth and sky coverage, XXL is particularly suited to systematically unveiling the clustering of X-ray clusters and to identifying superstructures in a homogeneous X-ray sample down to the typical mass scale of a local massive cluster. A friends-of-friends algorithm in three-dimensional physical space was run to identify large-scale structures. In this paper we report the discovery of the highest redshift supercluster of galaxies found in the XXL Survey. We describe the X-ray properties of the clusters members of the structure and the optical follow-up. The newly discovered supercluster is composed of six clusters of galaxies at a median redshift z around 0.43 and distributed across approximately 30 by 15 arc minutes (10 by 5 Mpc on sky) on the sky. This structure is very compact with all the clusters residing in one XMM pointing; for this reason this is the first supercluster discovered with the XXL Survey. Spectroscopic follow-up with WHT (William Herschel Telescope) and NTT (New Technology Telescope) confirmed a median redshift of z = 0.43. An estimate of the X-ray mass and luminosity of this supercluster and of its total gas mass put XLSSC-e at the average mass range of superclusters; its appearance, with two members of equal size, is quite unusual with respect to other superclusters and provides a unique view of the formation process of a massive structure.