Manolis Plionis

Minkowski functionals of Abell/ACO clusters

We determine the Minkowski functionals for a sample of Abell/ACO clusters, 401 with measured and 16 with estimated redshifts. The four Minkowski functionals (including the void probability function and the mean genus) deliver a global description of the spatial distribution of clusters on scales from

Cosmological constraints from the clustering properties of the X-ray Brightest Abell-type Cluster sample

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Clustering of galaxy clusters. II: Rare events in the cluster distribution

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Cluster versus POTENT density and velocity fields: cluster biasing and Ω

60hMpc

The Cluster Distribution as a Test of Dark Matter Models. III: The Cluster Velocity Field

with a clear geometric interpretation. Comparisons with mock catalogues of N--body simulations using different variants of the CDM model demonstrate the discriminative power of the description. The standard CDM model and the model with tilted perturbation spectrum cannot generate the Minkowski functionals of the cluster data, while a model with a cosmological constant and a model with breaking of the scale invariance of perturbations (BSI) yield compatible results.

The cluster distribution as a test of dark matter models – II. The dipole structure

We present an analysis of the two-point correlation function, ξ (r), of the X-ray Brightest Abell-type Cluster sample (XBACs) of Ebeling et al. and of the cosmological constraints that it provides. If ξ (r) is modelled as a power-law, ξ (r)=(r0/r)γ, we find r0≃ 26.0 ± 4.5 h-1 Mpc and γ≃ 2.0 ± 0.4, with errors corresponding to 2 σ uncertainties for one significant fitting parameter. As a general feature, ξ (r) is found to remain positive up to r ≃ 50–55 h-1 Mpc, after which it declines and crosses zero. Only a marginal increase of the correlation amplitude is found as the flux limit is increased from 5 × 10-12 to 12 × 10-12 erg s-1 cm-2, thus indicating a weak dependence of the correlation amplitude on the cluster X-ray luminosity. Furthermore, we present a method to predict correlation functions for flux-limited X-ray cluster samples from cosmological models. The method is based on the analytical recipe by Mo & White and on an empirical approach to convert cluster fluxes into masses. We use a maximum likelihood method to place constraints on the model parameter space from the XBACs ξ (r). For scale-free primordial spectra, we find that the shape parameter of the power spectrum is determined to lie in the 2 σ range 0.05 ≤Γλ 0.20. As for the amplitude of the power spectrum, we find σ8≃ 0.4–0.8 for Ο0=1 and σ8≃ 0.8–2.0 for Ο0=0.3. The latter result is in complete agreement with, although less constraining than, results based on the local cluster abundance.

Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 25,000 Kilometers per Second: The Bulk Velocity

Subsamples of the Abell and ACO cluster catalogs, which are nearly complete, are analyzed in order to study the large-scale structure traced by rich clusters. A variety of statistical techniques, minimal spanning trees, percolation, void probability functions, and cluster alignments, are used and the findings are compared with that expected from an ensemble of simulated cluster catalogs having the same selection functions and low-order clustering as the real data

Clustering of galaxy clusters. I: Is the spatial cluster-cluster correlation function enhanced significantly by contaminations ?

The density and velocity fields as extracted from the Abell/ACO clusters are compared with the corresponding fields recovered by the POTENT method from the Mark III peculiar velocities of galaxies. In order to minimize non-linear effects and to deal with ill-sampled regions, we smooth both fields using a Gaussian window with radii ranging between 12 and 20 h(-1) Mpc. The density and velocity fields within 70 h(-1) Mpc exhibit similarities, qualitatively consistent with gravitational instability theory and a linear biasing relation between clusters and mass. The random and systematic errors are evaluated with the help of mock catalogues. Quantitative comparisons within a volume containing similar to 12 independent samples yield beta(c)=Omega(0.6)/b(c)=0.22 +/- 0.08, where b(c) is the cluster biasing parameter at 15 h(-1) Mpc. If b(c)similar to 4.5, as indicated by the cluster correlation function, our result is consistent with Omega similar to 1.

The cluster distribution as a test of dark matter models – I. Clustering properties

We study the large-scale velocity fields traced by galaxy clusters in numerical simxad ulations of a box of side 960 h-1 Mpc, and compare them with available data on real clusters. In order to test the reliability of the simulations, which are based on an optimized version of the Zeldovich approximation, we compare their cluster velocities with those of exact N-body simulations, and find a remarkable agreement between the two according to a variety of statistical tests. We analyse cold dark matter (CDM) models with density parameter in the range 0.2 ::::; no ::::; 1, both with and without the cosmological constant term to provide a flat geometry. We also simulate a cold+hot dark matter (CHDM) model, with 30 per cent provided by the hot component. Comxad parison with real data is performed by applying tests based on the cumulative velocity frequency distribution (CVFD) and bulk flow statistics. For the CVFD, we use obserxad vational velocity data from different authors, and find that results based on different data sets are contradictory. In particular, the recent infrared Tully-Fisher (IRTF) data of Giovanelli yield smaller velocities with smaller errors than both the IR TF and Dn-<1 data of Hudson. It turns out that the Giovanelli data are only only consistent with the open no = 0.4 and the flat no = 0.2 models, while the Hudson data, though less discriminatory because of their larger errors, appear to exclude open models with no ::::; 0.4 and flat models with no = 0.2. This latter conclusion also holds if one pools all the data into a single sample regardless of the systematic differences in the two different sources. Furthermore, CVFD and bulk flow analyses of the Branchini et al. reconstructed velocity data again disfavour precisely those models accepted on the grounds of Giovanellis sample. Finally, we confirm that the Lauer & Postman reported bulk flow determination would be a rare event in the cosmological models we have analysed.

Multifractal analysis of cluster distribution in two dimensions

We study the dipole for a redshift sample of Abell/ACO clusters. To elucidate the constraints it places on dark matter models, we use numerical simulations based on the Zeldovich approximation. We run 20 realizations of each of six different dark matter models: four of these have a density parameter