X-ray Galaxy Clusters Research Articles

Constraining the Matter Power Spectrum Normalization Using the Sloan Digital Sky Survey/[ITAL]ROSAT[/ITAL] All-Sky Survey and REFLEX Cluster Surveys

We describe a new approach to constraining the amplitude of the power spectrum of matter perturbations in the universe, parameterized by σ8 as a function of the matter density Ω0. We compare the galaxy cluster X-ray luminosity function of the ROSAT-ESO Flux-Limited X-Ray (REFLEX) survey with the theoretical mass function of Jenkins et al., using the mass-luminosity relationship obtained from weak lensing data for a sample of galaxy clusters identified in Sloan Digital Sky Survey commissioning data and confirmed through cross-correlation with the ROSAT All-Sky Survey. We find σ8 = 0.38Ω, which is significantly different from most previous results derived from comparable calculations that used the X-ray temperature function. We discuss possible sources of systematic error that may cause such a discrepancy and in the process uncover a possible inconsistency between the REFLEX luminosity function and the relation between cluster X-ray luminosity and mass obtained by Reiprich & Bohringer.

TheROSAT‐ESO Flux‐limited X‐Ray (REFLEX) Galaxy Cluster Survey. IV. The X‐Ray Luminosity Function

The X-ray galaxy cluster sample from the REFLEX Cluster Survey, which covers the X-ray brightest galaxy clusters detected in the ROSAT All-Sky Survey in the southern sky, is used to construct the X-ray luminosity function of clusters in the local universe. With 452 clusters detected above an X-ray flux limit of 3 × 10-12 ergs s-1 cm-2 in 4.24 sr of the sky, this sample is the most comprehensive X-ray cluster sample with a well-documented selection function, providing the best current census of the local X-ray galaxy cluster population. In this paper we discuss the construction of the luminosity function and the effects of flux measurement errors and of variations with sample region, and we compare the results with those from previous surveys.

The Nearest Neighbor Alignment of Cluster X‐Ray Isophotes

We examine the orientations of rich galaxy cluster X-ray isophotes with respect to their rich nearest neighbors using existing samples of Abell cluster position angles measured from Einstein and ROSAT observations. We study a merged subset of these samples using updated and improved positions and redshifts for Abell/ACO clusters. We find high confidence for alignment, which increases as nearest neighbor distance is restricted. We conclude that there is a strong alignment signal in all this data, consistent with gravitational instability acting on Gaussian perturbation.

Cosmology from cosmic microwave background and galaxy clusters

We present the results of analysis of constraints on cosmological parameters from cosmic microwave background (CMB) alone and in combination with the galaxy cluster baryon fraction assuming inflation-generated adiabatic scalar fluctuations. The CMB constraints are obtained using our likelihood approximation method (Bartlett et al. 2000; Douspis et al. 2001). In the present analysis we use the new data coming from MAXIMA and BOOMERanG balloon-borne experiments and the first results of the DASI interferometer together with the COBE/DMR data. The quality of these independent data sets implies that the $C_\ell$ are rather well known, and allow reliable constraints. We found that the constraints in the $\Omega-H_0$ plane are very tightened, favouring a flat Universe, that the index of the primordial fluctuations is very close to one, that the primordial baryon density is now in good agreement with primordial nucleosynthesis. Nevertheless degeneracies between several parameters still exist, and for instance the constraint on the cosmological constant or the Hubble constant are very weak, preferred values being low. A way to break these degeneracies is to cross-constrain the parameters by combining them with constraints from other independent data. We use the baryon fraction determination from X-ray clusters of galaxies as an additional constraint and show that the combined analysis leads to strong constraints on all cosmological parameters. Using a high baryon fraction ( ∼ 15% for $h = 0.5$) we found a rather low Hubble constant, values around 80 km s -1 /Mpc being ruled out. Using a recent and low baryon fraction estimation ( ∼ 10% for $h = 0.5$) we found a preferred model with a low Hubble constant and a high density content ($\Omega_{\rm m}$), an Einstein-de Sitter model being only weakly ruled out.

Small-scale fluctuations of the soft X-ray background

We report the detection of the X-ray background fluctuations at small angular scales. ROSAT PSPC archive pointed observations are used to measure the autocorrelation function (ACF) of the background at scales of 0:03 0:3. The pointings have been selected from an area apparently free from galactic contamination and the ACF signal is generated mostly by the extragalactic component of the XRB. At separations below 0:1 known X-ray clusters of galaxies become a substantial source of the background fluctuations. Assuming a power law, the ACF for all the data has a slope of1, but is substantially flatter (with slope of0:6) when pointings containing bright clusters are removed. Flattening of the ACF at small separations could indicate existence of a hot, nonuniformly distributed gas accumulating in wells of gravitational potential. At separations 0:3 0: 4w here the ACF estimates based on the ROSAT pointings and All-Sky Survey are available, both data sets give consistent results.

The [ITAL]ROSAT[/ITAL] North Ecliptic Pole Survey X-Ray Data

In the ROSAT All-Sky Survey, an 80.7 deg2 region near the north ecliptic pole (NEP) constitutes the deepest contiguous observation of the X-ray sky. Four hundred and forty-five unique sources have been detected above a flux of ~2 × 10-14 ergs s-1 cm-2 (0.5-2.0 keV). The X-ray data for each source provide a likelihood of existence, position and position error, count rate and error, background count rate at the source position, exposure time, two hardness ratios and their errors, and a measure of extent and likelihood of nonzero extent. We describe the range and statistics of X-ray properties exhibited by the NEP sources and the accuracy with which they have been determined, concentrating on the flux measurement of extended galaxy cluster X-ray sources. We show two examples of sources whose X-ray morphology is useful in their optical identification.

The ROSAT-ESO flux limited X-ray (REFLEX) galaxy cluster survey. I. The construction of the cluster sample

We discuss the construction of an X-ray flux-limited sample of galaxy clusters, the REFLEX survey catalogue, to be used for cosmological studies. This cluster identification and redshift survey was conducted in the frame of an ESO key programme and is based on candidates selected from the southern part of the ROSAT All-Sky Survey (RASS). For the first cluster candidate selection from a flux-limited RASS source list, we make use of optical data from the COSMOS digital catalogue produced from the scans of the UK-Schmidt plates. To ensure homogeneity of the sample construction process, this selection is based only on this one well-defined optical data base. The nature of the candidates selected in this process is subsequently checked by a more detailed evaluation of the X-ray and optical source properties and available literature data. The final identification and the redshift is then based on optical spectroscopic follow-up observations. In this paper we document the process by which the primary cluster candidate catalogue is constructed prior to the optical follow-up observations. We describe the reanalysis of the RASS source catalogue which enables us to impose a proper flux limit cut to the X-ray source list without introducing a severe bias against extended sources. We discuss the correlation of the X-ray and optical (COSMOS) data to find galaxy density enhancements at the RASS X-ray source positions and the further evaluation of the nature of these cluster candidates. Based also on the results of the follow-up observations we provide a statistical analysis of the completeness and contamination of the final cluster sample and show results on the cluster number counts. The final sample of identified X-ray clusters reaches a flux limit of 3 10-12 erg s-1 cm-2 in the 0.1-2.4 keV band and comprises 452 clusters in an area of 4.24 ster. The results imply a completeness of the REFLEX cluster sample well in excess of 90% . We also derive for the first time an upper limit of less than 9% for the number of clusters which may feature a dominant contribution to the X-ray emission from AGN. This accuracy is sufficient for the use of this cluster sample for cosmological tests.

The power spectrum of flux-limited X-ray galaxy cluster surveys

We compute the redshift space power spectrum of two X-ray cluster samples: the X-ray Brightest Abell Cluster Sample (XBACS) and the Brightest Cluster Sample (BCS) using the method developed by Feldman, Kaiser & Peacock. The power spectrums derived for these samples are in agreement with determinations of other optical and X-ray cluster samples. For XBACS we find the largest power spectrum amplitude expected given the high richness of this sample ($R \ge 2$). In the range $0.05 \uk < k < 0.4 \uk$ the power spectrum shows a power law behavior $P(k)\propto k^{n}$ with an index $n\simeq-1.2$. In a similar range $0.04 \uk < k < 0.3 \uk $ BCS power spectrum has a smaller amplitude with index $n\simeq-1.0$. We do not find significant evidence for a peak at $k \simeq 0.05 \uk$ suggesting that claims such of feature detections in some cluster samples could relay on artificial inhomogeneities of the data. We compare our results with power spectrum predictions derived by Moscardini et al. within current cosmological models (LCDM and OCDM). For XBACS we find that both models underestimate the amplitude of the power spectrum but for BCS there is reasonably good agreement at $k\gsim 0.03 \uk$ for both models.

The ROSAT-ESO Flux-Limited X-Ray (REFLEX) galaxy cluster survey

We present a measure of the power spectrum on scales from 15 to 800h 1 Mpc using the ROSAT- ESO Flux-Limited X-Ray (REFLEX) galaxy cluster catalogue. The REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of galaxies with the nominal flux limit S =3 : 01 0 12 erg s 1 cm 2 for the ROSAT energy band (0:1 2:4) keV. Several tests are performed showing no signicant incompletenesses of the REFLEX clusters with X-ray luminosities brighter than 10 43 erg s 1 up to scales of about 800h 1 Mpc. They also indicate that cosmic variance might be more important than previous studies suggest. We regard this as a warning not to draw general cosmological conclusions from cluster samples with a size smaller than REFLEX. Power spectra, P (k), of comoving cluster number densities are estimated for flux- and volume-limited subsamples. The most important result is the detection of a broad maximum within the comoving wavenum- ber range 0:022 k 0:030h Mpc 1 . The data suggest an increase of the power spectral amplitude with X-ray luminosity. Compared to optically selected cluster samples the REFLEX P (k) is flatter for wavenumbers k 0:05h Mpc 1 thus shifting the maximum of P (k) to larger scales. The smooth maximum is not consis- tent with the narrow peak detected at k =0 :05h Mpc 1 using the Abell/ACO richness0 data. In the range 0:02 k 0:4h Mpc 1 general agreement is found between the slope of the REFLEX P (k) and those obtained with optically selected galaxies. A semi-analytic description of the biased nonlinear power spectrum in redshift space gives the best agreement for low-density Cold Dark Matter models with or without a cosmological constant.

Adaptive binning of X-ray galaxy cluster images

We present a simple method for adaptively binning the pixels in an image. The algorithm groups pixels into bins of size such that the fractional error on the photon count in a bin is less than or equal to a threshold value, and the size of the bin is as small as possible. The process is particularly useful for generating surface brightness and colour maps, with clearly defined error maps, from images with a large dynamic range of counts, for example X-ray images of galaxy clusters. We demonstrate the method in application to data from Chandra ACIS-S and ACIS-I observations of the Perseus cluster of galaxies. We use the algorithm to create intensity maps, and colour images that show the relative X-ray intensities in different bands. The colour maps can later be converted, through spectral models, into maps of physical parameters, such as temperature, column density, etc. The adaptive binning algorithm is applicable to a wide range of data, from observations or numerical simulations, and is not limited to two-dimensional data.

X-ray clusters of galaxies as tracers of structure in the Universe.

Clusters of galaxies are visible tracers of the network of matter in the Universe, marking the high-density regions where filaments of dark matter join together. When observed at X-ray wavelengths these clusters shine like cosmic lighthouses, as a consequence of the hot gas trapped within their gravitational potential wells. The X-ray emission is linked directly to the total mass of a cluster, and so can be used to investigate the mass distribution for a sizeable fraction of the Universe. The picture that has emerged from recent studies is remarkably consistent with the predictions for a low-density Universe dominated by cold dark matter.

-model and cooling flows in X-ray clusters of galaxies

The spatial emission from the core of cooling-flow clusters of galaxies is inadequately described by a β-model. Spectrally, the central region of these clusters is well approximated with a two-temperature model, where the inner temperature represents the multiphase status of the core and the outer temperature is a measure of the ambient gas temperature. Following this observational evidence, I extend the use of the β-model to a two-phase gas emission, where the two components coexist within a boundary radius rcool and the ambient gas alone fills the volume shell at a radius above rcool. This simple model still provides an analytic expression for the total surface brightness profile (Note in the first term the different sign with respect to the standard β-model.) Based upon a physically meaningful model for the X-ray emission, this formula can be used (i) to improve significantly the modelling of the surface brightness profile of cooling flow clusters of galaxies when compared to the standard β-model results, (ii) to constrain properly the physical characteristics of the intracluster plasma in the outskirts, like, e.g., the ambient gas temperature.

Cosmological Smoothed Particle Hydrodynamic Simulations with Four Million Particles: Statistical Properties of X‐Ray Clusters in a Low‐Density Universe

We present results from a series of cosmological smoothed particle hydrodynamics (SPH) simulations coupled with the particle-particle-particle-mesh solver for the gravitational force. The simulations are designed to predict the statistical properties of X-ray clusters of galaxies as well as to study the formation of galaxies. We have seven simulation runs with different assumptions about the thermal state of the intracluster gas. Following the recent work by Pearce and coworkers, we modify our SPH algorithm so as to phenomenologically incorporate galaxy formation by decoupling the cooled gas particles from the hot gas particles. All the simulations employ 1283 particles both for dark matter and for gas components and thus constitute the largest systematic catalogs of clusters simulated in the SPH method performed so far. These enable us to compare the analytical predictions about statistical properties of X-ray clusters against our direct simulation results in an unbiased manner. We find that the luminosities of the simulated clusters are quite sensitive to the thermal history and also to the numerical resolution of the simulations and thus are not reliable. On the other hand, the mass-temperature relation for the simulated clusters is fairly insensitive to the assumptions of the thermal state of the intracluster gas, is robust against the numerical resolution, and in fact agrees well with the analytic prediction. Therefore, prediction of the X-ray temperature functions of clusters on the basis of the Press-Schechter mass function and the virial equilibrium is fairly reliable.

A parallel TreeSPH code for galaxy formation

We describe a new implementation of a parallel TreeSPH code with the aim of simulating galaxy formation and evolution. The code has been parallelized using shmem, a Cray proprietary library to handle communications between the 256 processors of the Silicon Graphics T3E massively parallel supercomputer hosted by the Cineca Super-computing Center (Bologna, Italy).1 The code combines the smoothed particle hydrodynamics (SPH) method for solving hydrodynamical equations with the popular Barnes & Hut tree-code to perform gravity calculation with an N×log N scaling, and it is based on the scalar TreeSPH code developed by Carraro et al. Parallelization is achieved by distributing particles along processors according to a workload criterion. Benchmarks, in terms of load balance and scalability, of the code are analysed and critically discussed against the adiabatic collapse of an isothermal gas sphere test using 2×104 particles on 8 processors. The code results balance at more than the 95 per cent level. Increasing the number of processors, the load balance slightly worsens. The deviation from perfect scalability for increasing number of processors is almost negligible up to 32 processors. Finally, we present a simulation of the formation of an X-ray galaxy cluster in a flat cold dark matter cosmology, using 2×105 particles and 32 processors, and compare our results with Evrard's P3M–SPH simulations. Additionally we have incorporated radiative cooling, star formation, feedback from SNe of types II and Ia, stellar winds and UV flux from massive stars, and an algorithm to follow the chemical enrichment of the interstellar medium. Simulations with some of these ingredients are also presented.

The correlation function of X-ray galaxy clusters in the ROSAT All-Sky Survey 1 Bright Sample

We analyse the spatial clustering properties of the RASS1 Bright Sample, an X-ray flux-limited catalogue of galaxy clusters selected from the southern part of the ROSAT All-Sky Survey. The two-point correlation function �(r) of the whole sample is well fitted (in an Einstein-de Sitter model) by the power-law � = (r/r0) , with r0 = 21.5 +3.4 4.4 h 1 Mpc and = 2.11 +0.53 0.56 (95.4 per cent confidence level with one fitting parameter). We use the RASS1 Bright Sample as a first application of a theoretical model which aims at predicting the clustering properties of X-ray clusters in flux-limited surveys for different cosmological scenarios. The model uses the theoretical and empirical relations between mass, temperature and X-ray cluster luminosity, and fully accounts for the redshift evolution of the underlying dark matter clustering and cluster bias factor. The comparison between observational results and theoretical predictions shows that the Einstein-de Sitter models display too low a correlation length, while models with a matter density parameter 0m = 0.3 (with or without a cosmological constant) are successful in reproducing the observed clustering. The dependence of the correlation length r0 on the Xray limiting flux and luminosity of the sample is generally consistent with the predictions of all our models. Quantitative agreement is however only reached for 0m = 0.3 models. The model presented here can be reliably applied to future deeper X-ray cluster surveys: the study of their clustering properties will provide a useful complementary tool to the traditional cluster abundance analyses to constrain the cosmological parameters.

Cosmological implications of galaxy clusters in X-ray, millimeter, and submillimeter bands

Cosmological implications of clusters of galaxies are discussed with particular attention to their importance in probing the cosmological parameters. More specifically we compute the number counts of clusters of galaxies, Log N -Log S relation, in X-ray and submm bands on the basis of the Press-Schechter theory. As an important step toward breaking the degeneracy among the viable cosmological models, we observed the most luminous X-ray cluster RXJ1347-1145 in three bands (21 and 43 GHz in the Nobeyama Radio Observatory, Japan, and 350 GHz in the J. C. Maxwell telescope at Mauna Kea, Hawaii). We report on the preliminary results which are in good agreement with the profile of the Sunayev - Zel'dovich effect predicted on the basis of the previous X-ray observation of the cluster.

The Bright SHARC Survey: The X-Ray Cluster Luminosity Function

We present here initial results on the X-ray cluster luminosity function (XCLF) from the Bright Serendipitous High-Redshift Archival Cluster (SHARC) sample of distant X-ray clusters of galaxies. This sample is 97% complete in its optical identifications and contains 12 X-ray-luminous clusters in the redshift range 0.3 ≤ z ≤ 0.83 (median z = 0.42) and 1.1 × 1044 ≤ LX ≤ 8.3 × 1044 ergs s-1 (0.5-2.0 keV). We present a preliminary selection function for the Bright SHARC Survey based on Monte Carlo simulations. Using this selection function, we have computed the Bright SHARC Survey XCLF and find it to be fully consistent with a nonevolving XCLF to LX 5 × 1044 ergs s-1 and z 0.7. At LX > 5 × 1044 ergs s-1, we find evidence for a deficit of clusters compared to that expected from a nonevolving XCLF. We detect only one such cluster in the redshift range 0.3 ≤ z ≤ 0.7 when we would expect 4.9 clusters based on the local XCLF of De Grandi et al. The statistical significance of this deficit is 96%. To increase the statistical significance of this possible deficit, we have combined the Bright SHARC Survey and the 160 deg2 survey of Vikhlinin et al. This joint survey covers 260 deg2 and contains only one confirmed 0.3 ≤ z ≤ 0.7, LX > 5 × 1044 ergs s-1 cluster, while we would expect 7.6 such clusters based on the local XCLF (De Grandi et al.). The statistical significance of the deficit in this joint survey increases to 99.5%. These results remain preliminary because of incompletenesses in the optical follow-up and uncertainties in the local XCLF.

The empirical X-ray luminosity–gravitational mass relation for clusters of galaxies

A sample of one hundred and six X-ray galaxy clusters has been analysed in detail. Here we give the observed X-ray luminosity - gravitational mass relation. Possible future applications include conversion of empirical cluster luminosity functions to mass functions and conversion of simulated mass functions to luminosity functions.

X-ray galaxy clusters as cosmological probes

The X-ray properties of the galaxy cluster population are reviewed with evidence that clusters form a nearly self-similar class of objects. The use of the cluster X-ray luminosity function to constrain cosmological models and the assessment of the large-scale structure by means of the X-ray cluster distribution is discussed.

The empirical X-ray luminosity-gravitational mass relation for clusters of galaxies

A sample of one hundred and six X-ray galaxy clusters has been analysed in detail. Here we give the observed X-ray luminosity - gravitational mass relation. Possible future applications include conversion of empirical cluster luminosity functions to mass functions and conversion of simulated mass functions to luminosity functions.