arXiv subjects

S. Borgani

Publications and source records attributed to S. Borgani.

At least 181 records · Page 10Linked to original sources

arXiv2000

X-ray Clusters of Galaxies as Tracers of Structure in the Universe

Clusters of galaxies outline the network of the distribution of visible matter in the Universe, marking the highest-mass knots where filamentary structures join together. If we observe the sky in X rays, clusters of galaxies stand out as cosmic lighthouses by virtue of a thin gas trapped and heated within their gravitational potential wells. This powerful emission is directly linked to the total gravitating mass they contain, such that they can be efficiently used as tracers of the cosmic mass distribution within a sizeable fraction of the observable Universe. Recent observational campaigns have for the first time used X-ray clusters to map cosmic structures on scales approaching 3 billion light years. The emerging picture is remarkably consistent with the expectation of a low-density Universe dominated by cold dark matter.

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arXiv2000

Optical/Infrared Survey of Galaxy Clusters

In this contribution the ongoing effort to build a statistical sample of clusters of galaxies over a wide range of redshifts to study the evolution of clusters and member galaxies is reviewed. The starting point for this project has been the list of candidate clusters identified from the I-band EIS-WIDE survey data. Since the completion of this survey, new optical/infrared observations have become available and have been used to confirm some of these candidates using the photometric data alone or in combination with the results of follow-up spectroscopic observations. Our preliminary results show that the yield of real physical associations from the original catalog is conservatively >60% and that a large sample of clusters in the southern hemisphere, extending to high-redshifts, is within reach.

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arXiv2000

The Intracluster Medium in z > 1 Galaxy Clusters

The Chandra X-ray Observatory was used to obtain a 190 ks image of three high redshift galaxy clusters in one observation. The results of our analysis of these data are reported for the two z > 1 clusters in this Lynx field, including the most distant known X-ray selected cluster. Spatially-extended X-ray emission was detected from both these clusters, indicating the presence of hot gas in their intracluster media. A fit to the X-ray spectrum of RX J0849+4452, at z=1.26, yields a temperature of kT = 5.8^{+2.8}_{-1.7} keV. Using this temperature and the assumption of an isothermal sphere, the total mass of RX J0849+4452 is found to be 4.0^{+2.4}_{-1.9} X 10^{14} h_{65}^{-1} M_{\sun} within r = 1 h_{65}^{-1} Mpc. The T_x for RX J0849+4452 approximately agrees with the expectation based on its L_{bol} = 3.3^{+0.9}_{-0.5} X 10^{44}$ erg s$^{-1} according to the low redshift L_x - T_x relation. The very different distributions of X-ray emitting gas and of the red member galaxies in the two z > 1 clusters, in contrast to the similarity of the optical/IR colors of those galaxies, suggests that the early-type galaxies mostly formed before their host clusters.

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arXiv2000

Weak lensing mass reconstruction of MS1008.1-1224

We present an in-depth weak lensing analysis of the cluster MS1008 based on deep multicolor imaging obtained during the Science Verification of FORS1 at the VLT. The image quality (half arcsec seeing) and depth of the VLT images allow the shear signal to be mapped with high signal-to-noise and to be traced out to 1.2 h_50^-1 Mpc, near the edge of the 6'.8x6'.8 field of view. Using BVRI color information, as well as 81 redshifts in the field from the CNOC survey, background galaxies can be effectively separated from cluster and foreground objects. PSF distorsions are found to be moderate across the FORS images and thus easily removed. Due to the small statistical errors in the mass reconstruction, this dataset provides a testing ground where several systematic effects (e.g. mass-sheet degeneracy, redshift distribution of the background sources, cluster galaxy contamination) can be quantified. Several methods are used to remove the mass-sheet degeneracy which is found to dominate the systematic error budget. We measure a lower limit to the mass of 2.6 10^14 h_50^-1 M_sun within 1 h_50^-1 Mpc and a ``total'' mass of 5.3 10^14 h_50^-1 M_sun by fitting a softened isothermal sphere. The availability of the CNOC redshift data and X-ray observations on this cluster allow a comparison of different determinations of the mass radial profile. We find the lensing and X-ray measurements in excellent agreement, while the mass derived from the virial analysis is marginally (1-2 sigma) in agreement at radii where both methods are reliable.

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arXiv2000

Spectroscopic confirmation of clusters from the ESO imaging survey

We measure redshifts for 67 galaxies in the field of six cluster candidates from the ESO Imaging Survey (EIS). The cluster candidates are selected in the EIS patches C and D among those with estimated mean redshifts between 0.5 and 0.7. The observations were made with EFOSC2 at the 3.6m ESO telescope. In the six candidate cluster fields, we identify 19 possible sets of 2 to 7 galaxies in redshift space. In order to establish which of the 19 sets are likely to correspond to real dense systems we compare our counts with those expected from a uniform distribution of galaxies with given luminosity function. In order to take into account the effect of the Large Scale Structure, we modulate the probability computed from the luminosity function with random samplings of the Canada-France Redshift Survey. We find that four out of six candidate EIS clusters are likely to correspond to real systems in redshift space (> 95 % confidence level). Two of these systems have mean redshift in agreement with the redshift estimate given by the matched filter algorithm. The other two systems have significantly lower redshifts. We discuss the implications of our results in the context of our ongoing research projects aimed at defining high-redshift optically-selected cluster samples.

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arXiv2000

ENEAR Redshift-Distance Survey: Cosmological Constraints

We present an analysis of the ENEAR sample of peculiar velocities of elliptical galaxies, obtained with D_n-σdistances. We use the velocity correlation function to analyze the statistics of the field-object's velocities, while the analysis of the cluster data is based on the estimate of their rms peculiar velocity, Vrms. The statistics of the model velocity field is parameterized by the amplitude, η_8=σ_8 Ω_m^{0.6}, and by the shape parameter, Γ. From the velocity correlation statistics we obtain η_8=0.51{-0.09}{+0.24} for Γ=0.25 at the 2σlevel. Even though less constraining, a consistent result is obtained by comparing the measured Vrms of clusters to linear theory predictions. For Γ=0.25 we find η_8=0.63{-0.19}{+0.22}$ at 1σ. Overall, our results point toward a statistical concordance of the cosmic flows traced by spirals and early-type galaxies, with galaxy distances estimated using TF and D_n-σdistance indicators, respectively.

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arXiv2000

The Most Distant X-ray Clusters and the Evolution of their Space Density

We briefly review our current knowledge of the space density of distant X-ray clusters as measured by several ROSAT serendipitous surveys. We compare old and new determinations of the cluster X-ray Luminosity Function (XLF) at increasing redshifts, addressing the controversial issue of the evolution of its high end. We use complete subsamples, drawn from the ROSAT Deep Cluster Survey (RDCS), to quantify the statistical significance of the XLF evolution out to z ~1. A consistent observational picture emerges in which the bulk of the cluster population shows no significant evolution out to z ~1, whereas the most luminous systems (L_x >~ L* [0.5-2 keV] =~ 5x10^44 erg/s) were indeed rarer, at least at z >0.5, in keeping with the original findings of the EMSS. We also report on the recent spectroscopic identification of four clusters in the RDCS lying beyond z =1, the most distant X-ray clusters known to date, which set an interesting lower limit on the space density of clusters at z >1.

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arXiv1999

The ROSAT Deep Cluster Survey: Constraints on Cosmology

We use the ROSAT Deep Cluster Survey (RDCS) to trace the evolution of the cluster abundance out to $z\simeq 0.8$ and constrain cosmological models. We resort to a phenomenological prescription to convert masses into $X$-ray fluxes and apply a maximum-likelihood approach to the RDCS redshift- and luminosity-distribution. We find that, even changing the shape and the evolution on the $L_{bol}$-$T_X$ relation within the observational uncertainties, a critical density Universe is always excluded at more than $3σ$ level. By assuming a non-evolving $X$-ray luminosity-temperature relation with shape $L_{bol}\propto T_X^3$, it is $Ω_m=0.35^{+0.35}_{-0.25}$ and $σ_8=0.76^{+0.38}_{-0.14}$ for flat models, with uncertainties corresponding to $3σ$ confidence levels.

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arXiv1999

Confirming EIS Clusters. Multi-object Spectroscopy

Using EFOSC2 at the 3.6m ESO telescope, we obtained redshifts for 68 galaxies in the field of six cluster candidates from the ESO Imaging Survey (EIS). The cluster candidates were selected in the EIS patches C and D and have estimated mean redshifts between z=0.5 and z=0.7. In the six candidate cluster fields, we identify possible systems of galaxies in the redshift space. The likelihoods of these systems are established by comparison with random resamplings of the Canada-France Redshift Surveys, and using the redshift distribution expected from Postman et al.'s luminosity function, with Poggianti's K- and evolutionary-corrections. Four of the six candidate EIS clusters are found to correspond to a real system in the redshift space, with > 95 % probability. Two of them have a mean redshift in agreement with the estimate from the matched filter algorithm, while the other two have a significantly smaller redshift. The independent analysis of the V-I vs. I color-magnitude diagrams for five of our six cluster fields, supports our conclusions based on the spectroscopic data.

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arXiv1999

Correlation Analysis of SFI Peculiar Velocities

We present results of a statistical analysis of the SFI catalog of peculiar velocities, a recently completed survey of spiral field galaxies with I-band Tully-Fisher distances (Haynes et al. 1999). The velocity field statistic utilized is the velocity correlation function, $ψ_1(r)$ (Gorski et al. 1989). The analysis is performed in redshift space, so as to circumvent potential ambiguities connected with inhomogeneous Malmquist bias corrections. The results from the SFI sample are compared with linear-theory predictions. We generate a large set of mock samples, extracted from N-body simulations, which are used to assess the reliability of our analysis and to estimate the associated uncertainties. Defining $η_8=σ_8Ω_0^{0.6}$, we find that the measured $ψ_1(r)$ implies a degenerate constraint in the $η_8-Γ$ plane, with $η_8=(0.3 +/- 0.1) (Γ/0.2)^{0.5}$, at the $2σ$ level, for the inverse Tully-Fisher (ITF) calibration presented in this paper. Model constraints are quite sensitive to the ITF calibration. The other ITF calibrations by Giovanelli et al. (1997) and da Costa et al. (1998) both give, for $Γ=0.2$, $η_8\simeq 0.6$ as the best-fitting value.

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arXiv1999

Velocity Dispersions of CNOC Clusters and the Evolution of the Cluster Abundance

We present the results of the analysis of the internal velocity dispersions, σ_v, for the CNOC sample of distant galaxy clusters, based on an interlopers removal algorithm, which is different from that originally applied by Carlberg et al. (1996, C96). We find that the resulting σ_v values are consistent within <10% with the original C96 estimates. This result points in favor of a substantial robustness of currently applied methods for optical studies of the internal cluster dynamics. The resulting distribution of velocity dispersions is used to trace the redshift evolution of the cluster abundance with the aim of constraining the matter density parameter, Ω_m. We find that constraints on Ω_m are very sensitive to the adopted value of σ_8, as obtainable from the local cluster abundance: as σ_8 varies from 0.5 to 0.6 (for Omega_m=1), the best fitting Omega_m varies in the range 0.3-1.0.

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arXiv1999

Optical Luminosities and Mass--to--Light Ratios of Nearby Galaxy Clusters

We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well defined high degree of completeness. In particular, we consider a subsample of 89 clusters with B_j band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalogue. We compute cluster luminosities L_{B_j} within several clustercentric distances and within the virialization radius R_{vir}. In particular, we use the luminosity function and background counts estimated by Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, L_{B_j}, and galaxy velocity dispersion, sigma_v, with L_{B_j} proportional to sigma_v^{2.1--2.3}. Our estimate of typical value for the mass-to-light ratio is M/L_{B_j} about 250 (in solar units). We do not find any correlation of M/L_{B_j} with cluster morphologies, i.e. Rood--Sastry and Bautz--Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M proportional to L_{B_j}^{1.2--1.3}. We verify the robustness of this relation against a number of possible systematics.

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arXiv1999

Cosmological Constraints from the ROSAT Deep Cluster Survey

The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of X-ray selected galaxy clusters. Observables such as the flux number counts n(S), the redshift distribution n(z) and the X-ray luminosity function (XLF) over a large redshift baseline (z\lesssim 0.8) are used here in order to constrain cosmological models. Our analysis is based on the Press-Schechter approach, whose reliability is tested against N-body simulations. Following a phenomenological approach, no assumption is made a priori on the relation between cluster masses and observed X-ray luminosities. As a first step, we use the local XLF from RDCS, along with the high-luminosity extension provided by the XLF from the BCS, in order to constrain the amplitude of the power spectrum, σ_8, and the shape of the local luminosity-temperature relation. We obtain σ_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90% confidence level, almost independent of the L-T shape. The density parameter Ω_0 and the evolution of the L-T relation are constrained by the RDCS XLF at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions. By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an Ω_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at 90% confidence level, while Ω_0=0.4^{+0.3}_{-0.2} and Ω_0<0.6 are implied by a non--evolving L-T for open and flat models, respectively.

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arXiv1998

Cosmological Constraints from the Clustering Properties of the X-Ray Brightest Abell-type Cluster Sample

We present an analysis of the 2-point correlation function, of the X-ray Brightest Abell-type Cluster sample (XBACs; Ebeling et al. 1998) and of the cosmological constraints that it provides. If ξ(r) is modelled as a power-law, we find r_0=26.0 +/- 4.5 Mpc/h and γ=2.0 +/-0.4, with errors corresponding to 2σuncertainties. Only a marginal increase of the correlation amplitude is found as the flux limit is increased from 5x10^{-12} cgs to 12x10^{-12} cgs, 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 (1996) 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. We find that the shape parameter of the power spectrum lies in the 2σrange 0.05<Γ<0.20. As for the amplitude of the power-spectrum, we find σ_8 in the range 0.4-0.8 for Ω_0=1 and 0.8-2.0 for Ω_0=0.3. This result is in agreement with, although less constraining than, results based on the local cluster abundance.

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arXiv1998

Constraining cosmological models with cluster power spectra

Using extensive N-body simulations we estimate redshift space power spectra of clusters of galaxies for different cosmological models (SCDM, TCDM, CHDM, Lambda-CDM, OCDM, BSI, tau-CDM) and compare the results with observational data for Abell-ACO clusters. Our mock samples of galaxy clusters have the same geometry and selection functions as the observational sample which contains 417 clusters of galaxies in a double cone of galactic latitude |b| > 30 degrees up to a depth of 240 Mpc/h. The power spectrum has been estimated for wave numbers k in the range 0.03 < k < 0.2 h/Mpc. For k > k_max ~ 0.05 h/Mpc the power spectrum of the Abell-ACO clusters has a power-law shape, P(k)\propto k^n, with n ~ -1.9, while it changes sharply to a positive slope at k < k_max. By comparison with the mock catalogues SCDM, TCDM (n=0.9), and also OCDM with Omega_0 = 0.35 are rejected. Better agreement with observation can be found for the Lambda-CDM model with Omega_0 = 0.35 and h = 0.7 and the CHDM model with two degenerate neutrinos and Omega_HDM = 0.2 as well as for a CDM model with broken scale invariance (BSI) and the tau-CDM model. As for the peak in the Abell-ACO cluster power spectrum, we find that it does not represent a very unusual finding within the set of mock samples extracted from our simulations.

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arXiv1998

The Observational Mass Function of Nearby Galaxy Clusters

We present a new determination of the mass function of galaxy clusters, based on optical virial mass estimates for a large sample of 152 nearby (z\le0.15) Abell-ACO clusters, as provided by Girardi et al. (1998). This sample includes both data from the literature and the new ENACS data. The resulting mass function is reliably estimated for masses larger than M_{lim}\simeq 4\times 10^{14} \msun, while it is affected by sample incompleteness at smaller masses. We find N(>M_{lim})=(6.3\pm 1.2) 10^{-6} (h^{-1} Mpc)^{-3} for cluster masses estimated within a 1.5 h^{-1} radius. Our mass function is intermediate between the two previous estimates by Bahcall & Cen (1993) and by Biviano et al. (1993). Based on the Press-Schechter approach, we use this mass function to constrain the amplitude of the fluctuation power spectrum at the cluster scale. After suitably convolving the PS predictions with observational errors on cluster masses and COBE-normalizing the fluctuation power spectrum, we find sigma_8=(0.60\pm 0.04) Omega_0^{-0.46+0.09 Omega_0} for flat low-density models and sigma_8=(0.60\pm 0.04) Omega_0^{-0.48+0.17 Omega_0} for open models (at the 90% c.l.).

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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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