We study the distribution of X-ray selected clusters of galaxies with respect to superclusters determined by Abell clusters of galaxies and show that the distribution of X-ray clusters follows the supercluster-void network determined by Abell clusters. We find that in this network X-ray clusters are more strongly clustered than other clusters: the fraction of X-ray clusters is higher in rich superclusters, and the fraction of isolated X-ray clusters is lower than the fraction of isolated Abell clusters. There is no clear correlation between X-ray luminosity of clusters and their host supercluster richness. Poor, non-Abell X-ray clusters follow the supercluster-void network as well: these clusters are embedded in superclusters determined by rich clusters and populate filaments between them. We present a new catalog of superclusters of Abell clusters out to a redshift of z lim = 0.13, a catalog of X-ray clusters located in superclusters determined by Abell clusters, and a list of additional superclusters of X-ray clusters.

According to the favour models for the formation of large-scale structure in the Universe (in which the dynamics of the Universe is dominated by cold dark matter), the distribution of galaxies and clusters of galaxies should be random on large scales. It therefore came as a surprise when a periodicity was reported 1 in the distribution of high-density regions of galaxies in the direction of Galactic poles, although the appearent lack of periodicity in other directions led to the initial report being regarded as a statistical anomaly 2 . A subsequent study 3−6 also claimed evidence for periodicity on the same scale, but the statistical significance of this result was uncertain due to small number of clusters used. Here, using a new compilation 7 of available data on galaxy clusters, we present evidence for a quasiregular three-dimensional network of rich superclusters and voids, with the regions of high density separated by ∼ 120 Mpc. If this reflects the distribution of all matter (luminous and dark), then there must exists some hithero unknown process that produces regular structure on large scales.During the past few years the number of clusters with measured redshifts has increased considerably. To search for the possible presence of a regularity of the distribution of matter in the Universe we have used a new compilation 7 of available data on rich clusters of galaxies catalogued by Abell and collaborators 8,9 . The compilation has made use of all (∼300) published references on redshifts of both individual galaxies and Abell galaxy clusters. Individual galaxies were associated with a given Abell cluster if they lay within a projected distance of ≤1.5 h −1 Mpc (1 Abell radius) and within a factor of two of the redshift estimated from the brightness of the cluster's 10-th brightest galaxy, using the photometric estimate of Peacock & West 10 (h is the Hubble constant in units of 100 km/s/Mpc). The compilation contains measured redshifts for 869 of the 1304 clusters with an estimated redshift up to z = 0.12. For the present analysis we used all rich clusters (richness class R ≥ 0) in this compilation with at least two galaxy redshifts measured. The omission of the 435 clusters without measured redshifts does not affect our result because an appropriate selection function was used.This cluster sample (including clusters with estimated redshifts) was used to construct a new catalogue of 220 superclusters of galaxies 11,12 . These are systems of clusters where the distances between nearest neighbours among member clusters do not exceed 1

Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than m B = −20 is found to be smaller than about −0.8 by all our void finding algorithms.

We investigate the origin and evolution of fossil groups in a concordance ΛCDM cosmological simulation. We consider haloes with masses between 1 × 1013 and 5 × 1013 h−1 M⊙, and study the physical mechanisms that lead to the formation of the large gap in magnitude between the brightest and the second most bright group member, which is typical for these fossil systems. Fossil groups are found to have high dark matter concentrations, which we can relate to their early formation time. The large magnitude gaps arise after the groups have built up half of their final mass, due to merging of massive group members. We show that the existence of fossil systems is primarily driven by the relatively early infall of massive satellites, and that we do not find a strong environmental dependence for these systems. In addition, we find tentative evidence for fossil group satellites falling in on orbits with typically lower angular momentum, which might lead to a more efficient merger on to the host. We find a population of groups at higher redshifts that go through a ‘fossil phase’: a stage where they show a large magnitude gap, which is terminated by renewed infall from their environment.

Aims. We study the influence of the environment on the evolution of galaxies by investigating the luminosity function (LF) of galaxies of different morphological types and colours at different environmental density levels. Methods. We construct the LFs separately for galaxies of different morphology (spiral and elliptical) and of different colours (red and blue) using data from the Sloan Digital Sky Survey (SDSS), correcting the luminosities for the intrinsic absorption. We use the global luminosity density field to define different environments, and analyse the environmental dependence of galaxy morphology and colour. The smoothed bootstrap method is used to calculate confidence regions of the derived luminosity functions. Results. We find a strong environmental dependency for the LF of elliptical galaxies. The LF of spiral galaxies is almost environment independent, suggesting that spiral galaxy formation mechanisms are similar in different environments. Absorption by the intrinsic dust influences the bright-end of the LF of spiral galaxies. After attenuation correction, the brightest spiral galaxies are still about 0.5 mag less luminous than the brightest elliptical galaxies, except in the least dense environment, where spiral galaxies dominate the LF at every luminosity. Despite the extent of the SDSS survey, the influence of single rich superclusters is present in the galactic LF of the densest environment.