arXiv subjects

M. Plionis

Publications and source records attributed to M. Plionis.

106 records · Page 6Linked to original sources

arXiv1998

Large-Scale Coherent Dipole Anisotropy ?

We have reanalyzed and compared the dipoles of the 1.2 Jy and 0.6 Jy (QDOT) IRAS galaxy samples. We find strong indications from both samples for (a) significant contributions to the gravitational field that shapes the Local Group motion from depths up to $\sim 170$ $h^{-1}$ Mpc and (b) a large-scale coherence of the dipole anisotropy, indications provided mainly by the fact that the differential dipoles of large equal volume shells are aligned with the CMB dipole and exhibit significant dipole signals. The two IRAS dipoles are indistinguishable within 50 $h^{-1}$ Mpc and beyond $\sim 130$ $h^{-1}$ Mpc while the QDOT dipole, having a lower flux limit, continues growing with respect to the 1.2 Jy sample up to $\sim 100$ $h^{-1}$ Mpc in agreement with Rowan-Robinson et al (1990). The most probable cause of these deep dipole contributions is the Shapley mass concentration.

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arXiv1997

Modelling the X-ray cluster dipole and cluster contribution to the soft X-ray background

We investigate the sampling and dipole convergence properties of flux-limited samples of mock X-ray clusters in relation to their underlying ``parent'' cluster distribution. To this purpose, we resort to numerical simulations of the cluster distribution and extract samples resembling the main observational features of X-ray selected cluster samples. The flux-limited samples, being quite sparse, underestimate the amplitude of the ``parent'' cluster dipole by approximately 15 per cent on average for Local Group-like observers. However, the general shape of their dipole amplitude profiles are in relatively good agreement. We also calculate the expected contribution of clusters, selected according to the relevant criteria, to the soft (0.1-2.4 keV) extragalactic X-ray background, using the ESO Key Project X-ray luminosity function, assuming a flat universe with vanishing cosmological constant. We obtain a value of about 10 per cent of the observed XRB flux.

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arXiv1997

The Cluster Distribution as a Test of Dark Matter Models. IV: Topology and Geometry

We study the geometry and topology of the large-scale structure traced by galaxy clusters in numerical simulations of a box of side 320 $h^{-1}$ Mpc, and compare them with available data on real clusters. The simulations we use are generated by the Zel'dovich approximation, using the same methods as we have used in the first three papers in this series. We consider the following models to see if there are measurable differences in the topology and geometry of the superclustering they produce: (i) the standard CDM model (SCDM); (ii) a CDM model with $Ω_0=0.2$ (OCDM); (iii) a CDM model with a `tilted' power spectrum having $n=0.7$ (TCDM); (iv) a CDM model with a very low Hubble constant, $h=0.3$ (LOWH); (v) a model with mixed CDM and HDM (CHDM); (vi) a flat low-density CDM model with $Ω_0=0.2$ and a non-zero cosmological $Λ$ term ($Λ$CDM). We analyse these models using a variety of statistical tests based on the analysis of: (i) the Euler-Poincaré characteristic; (ii) percolation properties; (iii) the Minimal Spanning Tree construction. Taking all these tests together we find that the best fitting model is $Λ$CDM and, indeed, the others do not appear to be consistent with the data. Our results demonstrate that despite their biased and extremely sparse sampling of the cosmological density field, it is possible to use clusters to probe subtle statistical diagnostics of models which go far beyond the low-order correlation functions usually applied to study superclustering.

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arXiv1997

The X-ray Cluster Dipole

We estimate the dipole of the whole sky X-ray flux-limited sample of Abell/ACO clusters (XBACs) and compare it to the optical Abell/ACO cluster dipole. The X-ray cluster dipole is well aligned ($\le 25^{\circ}$) with the CMB dipole, while it follows closely the radial profile of its optical cluster counterpart although its amplitude is $\sim 10 - 30$ per cent lower. In view of the fact that the the XBACs sample is not affected by the volume incompleteness and the projection effects that are known to exist at some level in the optical parent Abell/ACO cluster catalogue, our present results confirm the previous optical cluster dipole analysis that there are significant contributions to the Local Group motion from large distances ($\sim 160h^{-1} $Mpc). In order to assess the expected contribution to the X-ray cluster dipole from a purely X-ray selected sample we compare the dipoles of the XBACs and the Brightest Cluster Sample (Ebeling et al. 1997a) in their overlap region. The resulting dipoles are in mutual good aggreement with an indication that the XBACs sample slightly underestimates the full X-ray dipole (by $\le 5$ per cent) while the Virgo cluster contributes about 10 - 15 per cent to the overall X-ray cluster dipole. Using linear perturbation theory to relate the X-ray cluster dipole to the Local group peculiar velocity we estimate the density parameter to be $β_{c_{x}} \simeq 0.24 \pm 0.05$.

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arXiv1996

Constraining the Power Spectrum using Clusters

(Shortened Abstract). We analyze a redshift sample of Abell/ACO clusters and compare them with numerical simulations based on the truncated Zel'dovich approximation (TZA), for a list of eleven dark matter (DM) models. For each model we run several realizations, on which we estimate cosmic variance effects. We analyse correlation statistics, the probability density function, and supercluster properties from percolation analysis. As a general result, we find that the distribution of galaxy clusters provides a constraint only on the shape of the power spectrum, but not on its amplitude: a shape parameter 0.18 < Γ< 0.25 and an effective spectral index at 20Mpc/h in the range [-1.1,-0.9] are required by the Abell/ACO data. In order to obtain complementary constraints on the spectrum amplitude, we consider the cluster abundance as estimated using the Press--Schechter approach, whose reliability is explicitly tested against N--body simulations. We conclude that, of the cosmological models considered here, the only viable models are either Cold+Hot DM ones with Ω_ν= [0.2-0.3], better if shared between two massive neutrinos, and flat low-density CDM models with Ω_0 = [0.3-0.5].

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arXiv1996

`Comparing the Predicted Abell/ACO Cluster & the MarkIII Galaxy Density and Velocity Fields', Contribution to the Wide Field Spectroscopy Meeting held in Athens, 20-24 May 1996

We compare the predicted Abell/ACO cluster density and velocity fields, using linear theory, with that based on the MarkIII catalog and the POTENT machinery within 7000 km/sec. Both density & velocity fields are in good qualitative agreement indicating that apart from a linear biasing factor the optical Abell/ACO clusters do trace the underlying mass distribution quite well.

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arXiv1996

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 $10$ to $60\hMpc$ 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.

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arXiv1995

Sampling Effects on Cosmological Dipoles

We use numerical simulations to investigate the behaviour of the dipole moment of the spatial distribution of different kinds of mass tracers. We select density peaks of the simulated matter distribution with mean separations of 38 and 30 $h^{-1}$ Mpc to represent two samples of rich clusters of galaxies. We extract, from the same simulations, samples selected to mimic the full 3D galaxy distribution of IR galaxies, and the flux--limited IRAS and QDOT galaxy samples. We compare the dipole moments of these ``galaxy'' and ``cluster'' samples in order to assess the effects of sampling uncertainties and shot--noise on the relationship between the ``true'' underlying galaxy dipole and the dipoles obtained for clusters and for the flux--limited galaxy samples. The results of this analysis demonstrate that the dipoles of both the IRAS and QDOT--like catalogues should trace the full 3D dipole shape fairly accurately, with the loss however of about 15--20$\%$ of the total 3D dipole amplitude. Furthermore, using a simple argument based on linear perturbation theory, on the linear biasing assumption and on the amplitude of the cluster dipole relative to that of galaxies, we can estimate their relative biasing factors quite accurately and in agreement with results obtained by other methods.

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arXiv1995

The Cluster Distribution as a Test for Dark Matter Models. II: The Dipole Structure

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 Zel'dovich approximation. We run 20 realizations of each of six different dark matter models: four of these have a density parameter $Ω_0=1$, while the other two have $Ω_0=0.2$, one with and one without a cosmological constant term. For the Abell/ACO sample, we evaluate the parameter $β=Ω_0^{0.6}/b_{ cl}$, which reaches its asymptotic value at $R=160 Mpc/h$. We find $β=0.15\pm 0.04$ and $0.25\pm 0.06$ in LG and CMB frames respectively. After identifying in the simulations LG-like observers, we construct mock cluster samples. We find that an alignment between the cluster dipole and observer velocity, such as that observed (about 20 degrees) should not be expected necessarily in the simulations. This, together with the large observer-to-observer variance estimates of $β$, makes it difficult to place any firm constraints on cosmological models. Our results demonstrate that the large amplitude and convergence depth of the cluster dipole cannot be taken as strong evidence either for or against a low--density Universe.

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arXiv1995

The Cluster Distribution as a Test for Dark Matter Models. I: Clustering Properties

We present simulations of the cluster distribution in several dark matter models, using an optimized version of the truncated Zel'dovich approximation (TZA). We compare them with N-body cluster simulations and find that the TZA provides a very accurate description of the cluster distribution as long as fluctuations on the cluster mass scale are in the mildly non-linear regime. The simulated dark matter models are: Standard CDM (SCDM), Tilted CDM (TCDM) with n=0.7, Cold+Hot DM (CHDM) with 30% of hot component, low Hubble constant (h=0.3) CDM (LOWH) and a spatially flat low-density CDM model with Ω_0=0.2. We compare the simulations with a redshift sample of Abell/ACO clusters, using the integral of the 2-point correlation function and the probability density function. We find that the best models at reproducing the data are CHDM and LCDM. All the other models are ruled out. The reduced skewness S_3 is fairly constant with S_3=1.9, independent of the DM model and consistent with observational data. The abundances of clusters predicted using the Press--Schechter theory provide strong constraints: only the CHDM, LOWH and LCDM models appear to produce the correct number-density of clusters.

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arXiv1995

Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 20000 km/sec. I: The Cluster 3D Dipole

Starting from the observed distribution of galaxy clusters in redshift space we use a two--step procedure to recover their distances and peculiar velocities. After statistically correcting for the unobserved cluster distribution in the zone of avoidance ($|b|\le 20^{\circ}$) and also for a smooth absorption at higher $|b|$'s, we use a dynamical iterative algorithm to recover the real--space cluster positions by minimizing the redshift space distortions. The whole procedure assumes that clusters trace the mass, that peculiar velocities are caused by gravity and that linear perturbation theory applies. The amplitude of the cluster dipole measured in the 3D space turns out to be $\sim 23\%$ less than that measured in redshift space. In both cases the dipole direction is aligned with the Cosmic Microwave Background dipole within $\sim 10^{\circ}$, taking into account the Virgocentric infall component of the Local Group motion. Observational errors, limitations in the reconstruction procedure and the intrinsic cosmological variance, which is the dominant source of uncertainty, render a stringent determination of the $β$ parameter whose central value turns out to be $β\approx 0.2$ while its total uncertainty is $\pm 0.1$. This implies that for a cluster-mass bias parameter of $\sim 5$, a flat Universe is not excluded, contrary to previous cluster-dipole $z$-space analysis.

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arXiv1994

Moments of the Cluster Distribution as a Test of Dark Matter Models

We estimate the variance and the skewness of the cluster distribution in several dark matter (DM) models. The cluster simulations are based on the Zel'dovich approximation, the low computational cost of which allows us to run 50 random realizations of each model. We compare our results with those coming from a similar analysis of a redshift sample of Abell/ACO clusters. Within the list of the considered models, we find that only the model based on Cold+Hot DM (with $Ω_{\rm hot}=0.3$) provides a good fit to the data. The standard CDM model and the low-density ($Ω_{\circ}=0.2$) CDM models, both with and without a cosmological constant term ($Ω_Λ=0.8$) are ruled out. The tilted CDM model with primordial spectral index $n=0.7$ and a low Hubble constant ($h=0.3$) CDM model are only marginally consistent with the data.

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arXiv1994

The Cluster Distribution as a Test for Dark Matter Models. I: Clustering Properties

We present extended simulations of the distribution of galaxy clusters in different dark matter models, using an optimized version of the Zel'dovich approximation. Six different dark matter models are studied: Standard CDM, Open CDM, Tilted CDM, Cold + Hot DM, low-density CDM with cosmological constant, and low Hubble constant ($h=0.3$) CDM. We compare simulations with an Abell/ACO redshift sample. We find that the models that best reproduce the clustering of the real data sets are the Cold + Hot DM model and the CDM model with a cosmological constant. The probability density function of all models and data is always well approximated by a lognormal distribution. We also find that the linear biasing parameter for the simulated clusters is nearly constant over a large range of scales, but its value depends on the model. We also note that the abundances of clusters predicted by the Press-Schechter theory provide strong constraints on these models: only the Cold + Hot DM model and the low-H CDM model appear to produce the correct number-density of clusters. Taking this constraint together with the cluster clustering statistics leads one to conclude that the best of our models is the Cold + Hot DM scenario.

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arXiv1994

The 1-Point Cluster Distribution Function and its Moments

We derive the 1-point probability density function of the smoothed 3-D Abell-ACO cluster density field and we compare it with that of artificial cluster samples, generated as high peaks of a Gaussian field in such a way that they reproduce the low-order (2- and 3-point) correlation functions and the observed cluster selection functions. We find that both real and simulated {\em pdf}'s are well approximated by a log-normal distribution even when the Gaussian smoothing radius is as large as 40 $h^{-1}$ Mpc. Furthermore the low-order moments of the {\em pdf} are found to obey a relation $γ= S_{3} σ^4$, with $γ$ being the skewness and $S_{3}\approx 1.8$. These results are consistent with clusters being high-peaks of an underlying initial Gaussian density field. A by-product of our analysis is that when we rescale the {\em pdf} cluster moments to those of the QDOT-IRAS galaxies, using linear biasing with $b_{cI}\sim 4.5$ and for the common smoothing radius of 20 $h^{-1}$ Mpc, we find them to be significantly smaller than those directly estimated from the QDOT data by Saunders et al. (1991).

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arXiv1994

Lick Galaxy Correlation Function Revised

We re-estimate the angular 2-point galaxy correlation function from the Lick galaxy catalogue. We argue that the large-scale gradients observed in the Lick catalogue are dominated by real clustering and therefore they should not be subtracted prior to the estimation of the 2-p correlation function. We find that if no such correction is introduced the galaxy correlations are perfectly consistent with the those found in the APM survey. Thus, the long standing discrepancy between the Lick and APM angular correlations is lifted.

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arXiv1993

Angular Distribution of Clustersin Skewed CDM Models

We perform a detailed investigation of the statistical properties of the projected distribution of galaxy clusters obtained in Cold Dark Matter (CDM) models with both Gaussian and skewed primordial density fluctuations. We use N-body simulations to construct a set artificial Lick maps. An objective cluster--finding algorithm is used to identify clusters of different richness. For Gaussian models, the overall number of clusters is too small in the standard CDM case, but a model with higher normalisation fares much better; non--Gaussian models with negative skewness also fit faily well. We apply several statistical tests to compare real and simulated cluster samples, such as the 2-point correlation function, the minimal spanning tree construction, the multifractal analysis and the skewness of cell counts. The emerging picture is that Gaussian models, even with a higher normalization, are in trouble. Skew-positive models are also ruled out, while skew-negative models can reproduce the observed clustering of galaxy clusters in the CDM framework.

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