arXiv subjects

Manolis Plionis

Publications and source records attributed to Manolis Plionis.

53 records · Page 3Linked to original sources

arXiv2005

Cosmological constrains from X-ray AGN clustering and SNIa data

We put constraints on the main cosmological parameters of different spatially flat cosmological models by combining the recent clustering results of XMM-Newton soft (0.5-2 keV) X-ray sources, which have a redshift distribution with median redshift $z\sim 1.2$, and SNIa data. Using a likelihood procedure we find that the model which best reproduces the observational data and that is consistent with stellar ages is the concordance $Λ$CDM model with: $Ω_{\rm m}\simeq 0.28$, $w\simeq -1$, $H_{\circ} \simeq 72$ km s$^{-1}$ Mpc$^{-1}$, $t_{\circ}\simeq 13.5$Gyr and has an X-ray AGN clustering evolution which is constant in physical coordinates. For a different clustering evolution model (constant in comoving coordinates) we find another viable model, although less probable due to the smaller age of the Universe, with $Ω_{\rm m}\simeq 0.38$, $w\simeq -1.25$, $H_{\circ} \simeq 70$ km s$^{-1}$ Mpc$^{-1}$, $t_{\circ}\simeq 12.9$Gyr.

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arXiv2004

The Large-Scale Environment of Groups & Clusters of Galaxies

It appears that the dynamical status of clusters and groups of galaxies is related to the large-scale structure of the Universe. A few interesting trends have been established: (1) The Cluster Substructure - Alignment Connection, by which clusters show a strong correlation between their tendency to be aligned with their neighbors and their dynamical state (as indicated by the existence of significant substructres). (2) The Cluster Dynamics -Cluster Clustering Connection, by which dynamically young clusters are more clustered than the overall cluster population. (3) The Cluster- Supercluster Alignment Connection, by which clusters of galaxies show a statistical significant tendency to be aligned with the projected major axis orientation of their parent supercluster. (4) The Galaxy Alignment - Cluster Dynamics Connection, by which red-sequence cluster bright galaxies show a significant trend to be aligned with their parent cluster major axis, especially in dynamically young clusters. (5) The Group Richness - Shape Connection, by which groups of galaxies are flatter the poorer they are. These are strong indications that clusters develop in a hierarchical fashion by anisotropic merging of smaller units along the large-scale filamentary structures within which they are embedded.

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arXiv2004

The Richness-Shape Relation of the 2dFGRS Groups

We estimate the group shape of the recently compiled Pecolation-Inferred Galaxy Group (2PIGG) catalogue. Using a set of integral equations we invert the projected distribution of group axial ratios and recover the corresponding intrinsic distribution under the assumption that groups are pure spheroids. In agreement with the analysis of the UZC-SSRS2 group sample (Plionis, Basilakos & Tovmassian 2004) we find that groups are extremely elongated prolate-like systems. We also find an interesting trend between shape and group richness with poorer groups being significantly flatter than richer ones.

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arXiv2002

Recent Dynamical Evolution of Galaxy Clusters

Evidence is presented for a recent evolution of the relaxation processes in clusters of galaxies, using large optical and X-ray cluster samples. The criteria of the cluster relaxation used are the cluster ellipticity, the ICM temperature and X-ray cluster luminosity. We find evidence of varying strength and significance of all three indicators evolving with redshift for z <0.15. This result supports the view that clusters have mostly stopped undergoing mergers and accreting matter, as expected in a low-Omega_matter Universe, and are now in the process of gravitational relaxation, which reduces their flattening, their ICM temperature (shock heated during the merging phase), and their X-ray luminosity. These results support similar recent claims of Melott, Chambers and Miller (2001).

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arXiv2001

The Substructure-Alignment Connection

Using a sample of 903 APM clusters we investigate whether their dynamical status, as evidenced by the presence of significant substructures, is related to the large-scale structure of the Universe. We find that the cluster dynamical activity is strongly correlated with the tendency of clusters to be aligned with their nearest neighbour and in general with the nearby clusters that belong to the same supercluster. Furthermore, dynamically active clusters are more clustered than the overall cluster population. These are strong indications that clusters develop in a hierarchical fashion by anisotropic merging along the large-scale filaments within which they are embedded.

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arXiv2001

Large-Scale Environmental Effects of the Cluster Distribution

Using the APM cluster distribution we find interesting alignment effects: (1) Cluster substructure is strongly correlated with the tendency of clusters to be aligned with their nearest neighbour and in general with the nearby clusters that belong to the same supercluster, (2) Clusters belonging in superclusters show a statistical significant tendency to be aligned with the major axis orientation of their parent supercluster. Furthermore we find that dynamically young clusters are more clustered than the overall cluster population. These are strong indications that cluster develop in a hierarchical fashion by merging along the large-scale filamentary superclusters within which they are embedded.

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arXiv2001

Cosmological Implications of the PSCz PDF and its Moments

We compare the {\em pdf} and its low-order moments (variance and skewness) of the smoothed PSCz galaxy density field and of the corresponding simulated PSCz look-alikes, generated from N-body simulations of 6 different DM models; four structure normalized with $Γ=0.25$ and $σ_{8}=0.55 Ω_{\circ}^{-0.6}$, one COBE normalized and the old SCDM. The galaxy distributions are smoothed with a Gaussian window at three different smoothing scales $R_{sm}=5$, 10 and $15 \hm$. We find that the simulation PSCz look-alike {\em pdf}'s are sensitive only on the normalization of the power spectrum, probably due to shape similarity of the simulated galaxy power spectrum on the relevant scales. We find that the only models that are consistent, at a high significance level, with the observed PSCz {\em pdf} are models with a relatively low power spectrum normalization $(σ_{8}=0.83)$. From the phenomenologically derived $σ_{8}-moments$ relation, fitted from the simulation data, we find that the PSCz moments suggest $σ_{8} \simeq 0.7$.

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arXiv2001

The Cluster Substructure - Alignment Connection

Using the APM cluster data we investigate whether the dynamical status of clusters is related to the large-scale structure of the Universe. We find that cluster substructure is strongly correlated with the tendency of clusters to be aligned with their nearest neighbour and in general with the nearby clusters that belong to the same supercluster. Furthermore, dynamically young clusters are more clustered than the overall cluster population. These are strong indications that cluster develop in a hierarchical fashion by anisotropy merging along the large-scale filamentary superclusters within which they are embedded.

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arXiv2000

Cosmological Evolution of Linear Bias

Using linear perturbation theory and the Friedmann-Lemaitre solutions of the cosmological field equations, we derive analytically a second-order differential equation for the evolution of the linear bias factor, b(z), between the background matter and a mass-tracer fluctuation field. We find b(z) to be a strongly dependent function of redshift in all cosmological models. Comparing our analytical solution with the semi-analytic model of Mo & White, which utilises the Press-Schechter formalism and the gravitationally induced evolution of clustering, we find an extremely good agreement even at large redshifts, once we normalize to the same bias value at two different epochs, one of which is the present. Furthermore, our analytic b(z) function agrees well with the outcome of N-body simulations even up to large redshifts.

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arXiv2000

Searching for cluster substructure using APM and ROSAT data

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell+APM) data with X-ray (0.1 - 2.4) keV ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, center-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good 1-to-1 correspondence between the optical and the X-ray images and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order 45 per cent which is in very good accordance with other similar analyses. Finally, 5 out of 22 systems seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.

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arXiv2000

The Apparent and Intrinsic Shape of the APM Galaxy Clusters

We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of their apparent shapes. We measure the projected cluster ellipticities using two alternative methods. The first method is based on moments of the discrete galaxy distribution while the second is based on moments of the smoothed galaxy distribution. We study the performance of both methods using Monte Carlo cluster simulations covering the range of APM cluster distances and including a random distribution of background galaxies. We find that the first method suffers from severe systematic biases, whereas the second is more reliable. After excluding clusters dominated by substructure and quantifying the systematic biases in our estimated shape parameters, we recover a corrected distribution of projected ellipticities. We use the non-parametric kernel method to estimate the smooth apparent ellipticity distribution, and numerically invert a set of integral equations to recover the corresponding distribution of intrinsic ellipticities under the assumption that the clusters are either oblate or prolate spheroids. The prolate spheroidal model fits the APM cluster data best.

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arXiv2000

Testing for substructure in optical and X-ray clusters

We present a detailed study of the morphological features of 22 rich galaxy clusters. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and X-ray images. To this end, we compute moments of the surface-brightness distribution to estimate ellipticities, center-of-mass shifts and orientations. We find important correlations between the optical and X-ray morphological shape parameters. Most of our clusters (17) have a good 1-to-1 correspondence between the optical and the X-ray images and at least 9 appear to have strong indications of substructure. This corresponds to a percentage of about 40% in good accordance with other similar analyses. Finally, 4 out of 22 systems seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. We assess the significance of results using Monte Carlo simulations.

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arXiv1999

Galaxy Cluster Shapes

We estimate the distribution of intrinsic shapes of the APM galaxy clusters from their corresponding distribution of projected shapes. We smooth the discrete galaxy distribution and define the cluster shape by fitting the best ellipse to the different isodensity contours. Using Monte-Carlo simulations we have studied the performance of our method in the presence of the expected galaxy background, at the different distances traced by the APM clusters, and we have devised a method to correct statistically the projected cluster shapes for discreteness effects and random fluctuations. We find that the true cluster shape is consistent with that of prolate spheroids.

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arXiv1995

Reconstructing Positions \& Peculiar Velocities of Galaxy Clusters within 25000 km/sec: The Bulk Velocity

Using a dynamical 3-D reconstruction procedure we estimate the peculiar velocities of $R\ge0$ Abell/ACO galaxy clusters from their measured redshift within 25000 km/sec. The reconstruction algorithm relies on the linear gravitational instability hypothesis, assumes linear biasing and requires an input value of the cluster $β$-parameter ($β_c \equiv Ω_{\circ}^{0.6}/b_c$), which we estimated in Branchini \& Plionis (1995) to be $β_c\simeq 0.21$. The resulting cluster velocity field is dominated by a large scale streaming motion along the Perseus Pisces--Great Attractor base-line directed towards the Shapley concentration, in qualitative agreement with the galaxy velocity field on smaller scales. Fitting the predicted cluster peculiar velocities to a dipole term, in the local group frame and within a distance of $\sim 18000$ km/sec, we recover extremely well both the local group velocity and direction, in disagreement with the Lauer \& Postman (1994) observation. However, we find a $\sim 6\%$ probability that their observed velocity field could be a realization of our corresponding one, if the latter is convolved with their large distance dependent errors. Our predicted cluster bulk velocity amplitude agrees well with that deduced by the POTENT and the da Costa et al. (1995) analyses of observed galaxy motions at $\sim 5000 - 6000$ km/sec; it decreases thereafter while at the Lauer \& Postman limiting depth ($\sim 15000$ km/sec) its amplitude is $\sim 150$ km/sec, in comfortable agreement with most cosmological models.

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arXiv1995

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

We study the large-scale velocity fields traced by galaxy clusters in numerical simulations 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 Zel'dovich 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 models with density parameter in the range $0.2\le Ω_0\le 1$, both with and without the cosmological constant term to provide a flat geometry. We also simulate a Cold+Hot Dark Matter model, with 30% provided by the hot component. Comparison 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 observational velocity data from different authors. By merging all available data in a combined sample, the CVFD test is able to exclude only open models with $Ω_0\le 0.4$ and flat models with $Ω_0=0.2$. However, the analysis of individual observational samples gives contradictory results; in particular the recent Giovanelli data, which imply much lower velocities (with significantly smaller uncertainties) than the other data, are actually only consistent with the open $Ω_0=0.4$ and the flat $Ω_0=0.2$ models, i.e. the models which are excluded by the other data. Furthermore, CVFD and bulk flow analyses of the Branchini et al. reconstructed velocity data disfavour precisely those models accepted on the grounds of Giovanelli's sample. Finally, we confirm that the Lauer & Postman bulk flow determination is an extremely rare event in the cosmological models we have analysed.

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arXiv1995

THE VALUE OF βFROM THE LG AND ABELL CLUSTER PECULIAR VELOCITIES

Comparing the observed peculiar velocities of the Local Group and of nearby Abell clusters of galaxies with those predicted using linear perturbation theory and the 3-D reconstruction algorithm of Branchini \& Plionis (1995) the value of $β$ can be estimated. I show that the anisotropy that causes the LG motion does not only extend to very large depths ($\sim 160 \; h^{-1}$ Mpc) but it is also very coherent, which suggests that the value of $β\approx 0.21 \pm 0.015$, obtained from the LG velocity, should not be significantly affected by cosmic variance. Comparing the Abell cluster and QDOT galaxy dipoles in real space their relative biasing factor are estimated to be $b_{cI}\approx 3.5\pm0.5$. Finally, using a suitable set of available cluster peculiar velocities I obtain a somewhat lower value of $β\approx 0.15 - 0.17$ but with a large uncertainty ($\gtrsim 0.1$) when weighted by the large observational errors.

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arXiv1993

Position Angles and Alignments of Galaxy Clusters

The position angles of a large number of Abell and Shectman clusters (in total 637 clusters), identified in the Lick map as surface density enhancements, are estimated. Using published redshifts for 277 of these I have detected strong nearest neighbour alignments (2.5 - 3 sigma level), up to about 15 Mpc (Ho = 100). Weaker alignments but still significant are found up to larger separations. An even more significant alignment (4 sigma) is found between all neighbours residing in superclusters and having separations <= 10 Mpc. Again weaker but significant alignments are found when larger separations are considered. I have performed various tests to check whether systematic effects could induce an artificial alignment signal but I find no evidence for the presence of such effects. Since my cluster sample is neither volume limited nor redshift complete, a fact that would tend to wash-out any real alignment signal, the alignments detected should reflect a real and possibly a stronger underline effect.

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