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Spyros Basilakos

Publications and source records attributed to Spyros Basilakos.

142 records · Page 8Linked to original sources
arXiv2006

Virialization of cosmological structures in models with time varying equation of state

We study the virialization of the cosmic structures in the framework of flat cosmological models where the dark energy component plays an important role in the global dynamics of the universe. In particular, our analysis focuses on the study of the spherical matter perturbations, as the latter decouple from the background expansion and start to ``turn around'' and finally collapse. We generalize this procedure taking into account models with an equation of state which vary with time, and provide a complete formulation of the cluster virialization attempting to address the nonlinear regime of structure formation. In particular, assuming that clusters have collapsed prior to the epoch of $z_{\rm f}\simeq 1.4$, in which the most distant cluster has been found, we show that the behavior of the spherical collapse model depends on the functional form of the equation of state.

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arXiv2006

The PSCz Dipole Revisited

We re-examine the gravitational acceleration (dipole) induced on the Local Group of galaxies by the Infrared Astronomical Satellite (IRAS) galaxy distribution of the Point Source Catalogue redshift survey (PSCz). We treat the cirrus-affected low galactic latitudes by utilizing a spherical harmonic expansion of the galaxy surface density field up to the octapole order. We find strong indications for significant contributions to the Local Group motion from depths up to $\sim 185$ $h^{-1}$ Mpc and possible contribution even from $\sim 210$ $h^{-1}$ Mpc, in agreement with the recent analysis of Kocevski & Ebeling of a whole sky X-ray cluster survey.What changes with respect to the previous PSCz dipole analyses is: (a) the large-scale dipole contributions and (b) an increase of the overall dipole amplitude due to the important contribution of the local volume ($\mincir 4 h^{-1}$Mpc), which we now take into account. This results in a lower value of the $β(\equiv Ω_{\rm m}^{0.6}/b)$ parameter, which we find to be $β_{\rm IRAS} \simeq 0.49$ in real space. Therefore, for the concordance cosmological model ($Ω_{\rm m}=1-Ω_Λ=0.3$) the IRAS galaxies bias factor is $b_{\rm IRAS}\simeq 1$ which means that IRAS galaxies are good traces of the underlying matter distribution.

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arXiv2006

Constraining the CDM spectrum normalization in flat dark energy cosmologies

We study the relation between the rms mass fluctuations on 8$h^{-1}$Mpc scales and $Ω_{\rm m}$ using the recent clustering results of XMM-{\it Newton} soft (0.5-2 keV) X-ray sources, which have a median redshift of $z\sim 1.2$. The relation can be represented in the form $σ_{8}=0.34 (\pm 0.01) Ω_{\rm m}^{-γ}$ where $γ\equiv γ(Ω_{\rm m},w)$ and it is valid for all $w<-1/3$ models. By combining the X-ray clustering and SNIa data we find that the model which best reproduces the observational data is that with: $Ω_{\rm m}\simeq 0.26$, $w\simeq -0.90$ and $σ_{8}\simeq 0.73$, which is in excellent agreement with the recent 3-year Wilkinson Microwave Anisotropy Probe results.

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arXiv2006

Halo Shapes, Dynamics and Environment

In the hierarchical structure formation model cosmic halos are supposed to form by accretion of smaller units along anisotropic direction, defined by large-scale filamentary structures. After the epoch of primary mass aggregation (which depend on the cosmological model), violent relaxation processes will tend to alter the halo phase-space configuration producing quasi-spherical halos with a relatively smooth density profiles. Here we attempt to investigate the relation between halos shapes, their environment and their dynamical state. To this end we have run a large ($L=500 h^{-1}$ Mpc, $N_{p}=512^3$ particles) N-body simulation of a flat low-density cold dark matter model with a matter density $Ω_{\rm m}=1-Ω_Λ=0.3$, Hubble constant $H_{\circ}=70$ km s$^{-1}$ Mpc$^{-1}$ and a normalization parameter of $σ_{8}=0.9$. The particle mass is $m_{\rm p}\ge 7.7\times 10^{10} h^{-1} M_{\odot}$ comparable to the mass of one single galaxy. The halos are defined using a friends-of-friend algorithm with a linking length given by $l=0.17\barν$ where $\barν$ is the mean density. This linking length corresponds to an overdensity $ρ/ρ_{\rm mean}\simeq 200$ at the present epoch ($z=0$) and the total number of halos with more than 130 particles ($M>3 \times 10^{13} h^{-1} M_{\odot}$) is 57524.

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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 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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arXiv2003

Shape Statistics of Sloan Digital Survey superclusters

We study the supercluster shape properties of the recently compiled SDSS cluster catalog using an approach based on differential geometry. We detect superclusters by applying the percolation algorithm to observed cluster populations, extended out to $z_{\rm max}\leq 0.23$ in order to avoid selection biases. We utilize a set of shapefinders in order to study the morphological features of superclusters with $\geq 8$ cluster members and find that filamentary morphology is the dominant supercluster shape feature, in agreement with previous studies.

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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

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

Constraints on Cosmological and Biasing models using AGN clustering

We attempt to put constraints on different cosmological and biasing models by combining the recent clustering results of X-ray sources in the local ($z\le 0.1$) and distant universe ($z\sim 1$). To this end we compare the measured angular correlation function for bright (Akylas et al. 2000) and faint (Vikhlinin & Forman 1995) {\em ROSAT} X-ray sources respectively with those expected in three spatially flat cosmological models. Taking into account the different functional forms of the bias evolution, we find that there are two cosmological models which performs well the data. In particular, low-$Ω_{\circ}$ cosmological models ($Ω_Λ=1-Ω_{\circ}=0.7$) which contain either (i) high $σ_{8}^{\rm mass}=1.13$ value with galaxy merging bias, $b(z) \propto (1+z)^{1.8}$ or (ii) low $σ_{8}^{\rm mass}=0.9$ with non-bias, $b(z) \equiv 1$ best reproduce the AGN clustering results. While $τ$CDM models with different bias behaviour are ruled out at a high significance level.

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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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