Sample Of X-ray Clusters Research Articles

A volume-limited sample of X-ray galaxy groups and clusters – I. Radial entropy and cooling time profiles

We present the first results of our study of a sample of 101 X-ray galaxy groups and clusters, which is volume-limited in each of three X-ray luminosity bins. The aim of this work is to study the properties of the innermost ICM in the cores of our groups and clusters, and to determine the effect of non-gravitational processes, such as active galactic nucleus (AGN) feedback, on the ICM. The entropy of the ICM is of special interest, as it bears the imprint of the thermal history of a cluster, and it also determines a cluster's global properties. Entropy profiles can therefore be used to examine any deviations from cluster self-similarity, as well as the effects of feedback on the ICM. We find that the entropy profiles are well-fitted by a simple powerlaw model, of the form $K(r) = \alpha\times(r/100 \rm{kpc})^{\beta}$, where $\alpha$ and $\beta$ are constants. We do not find evidence for the existence of an "entropy floor", i.e. our entropy profiles do not flatten out at small radii, as suggested by some previous studies.

Statistics and implications of substructure detected in a representative sample of X-ray clusters

We present a morphological study of 35 X-ray luminous galaxy clusters at 0.15<z<0.3, selected in a similar manner to the Local Cluster Substructure Survey (LoCuSS), for which deep XMM-Newton observations are available. We characterise the structure of the X-ray surface brightness distribution of each cluster by measuring both their power ratios and centroid shift, and thus rank the clusters by the degree of substructure. These complementary probes give a consistent description of the cluster morphologies with some well understood exceptions. We find a remarkably tight correlation of regular morphology with the occurrence of cool cores in clusters. We also compare our measurements of X-ray morphology with measurements of the luminosity gap statistics and ellipticity of the brightest cluster galaxy (BCG). We check how our new X-ray morphological analysis maps onto cluster scaling relations, finding that (i) clusters with relatively undisturbed X-ray morphologies are on average more luminous at fixed X-ray temperature than those with disturbed morphologies, and (ii) disturbed clusters have larger X-ray masses than regular clusters for a given temperature in the M-T relation. We also show that the scatter in the ratio of X-ray and weak lensing based cluster mass measurements is larger for disturbed clusters than for those of more regular morphology. Overall, our results demonstrate the feasibility of assembling a self-consistent picture of the physical structure of clusters from X-ray and optical data, and the potential to apply this in the measurement of cosmological cluster scaling relations.

The influence of mergers on the cluster temperature function and cosmological parameters derived from it

We develop a parameter-free analytic model to include the effects of mergers into the theoretical modelling of the X-ray temperature function of galaxy clusters. We include this description into our model for the cluster population based on fluctuations of the gravitational potential, which avoids any reference to mass. Comparisons with a numerical simulation reveal that the theoretical model is in good agreement with the simulation results. We show that building the model on the dynamics of spherical rather than ellipsoidal collapse yields better results if emission-weighted temperatures are used, while ellipsoidal collapse yields good agreement between model and simulation for mass-weighted temperatures. Analysing two different samples of X-ray clusters, we quantify the influence of mergers and a conversion between different temperature definitions on the joint determination of Omega_m0 and sigma_8. If effects of mergers are included, temperature functions based on cluster masses and on the gravitational potential built on spherical collapse are in good agreement with other cosmological probes without any conversion of temperatures.

Measuring redshifts using X-ray spectroscopy of galaxy clusters: results fromChandradata and future prospects

The ubiquitous presence of the Fe line complex in the X-ray spectra of galaxy clusters offers the possibility of measuring their redshift without resorting to spectroscopic follow-up observations. In this paper we assess the accuracy with which the redshift of galaxy clusters can be recovered from an X-ray spectral analysis of Chandra archival data. This study indicates a strategy to build large surveys of clusters whose identification and redshift measurement are both based on X-ray data alone. We apply a blind search for K--shell and L--shell Fe line complex in X-ray cluster spectra using Chandra archival observations of galaxy clusters. The Fe line in the ICM spectra can be detected by simply analyzing the C-statistics variation $\Delta C_{stat}$ as a function of the redshift parameter. We repeat the measurement under different conditions, and compare the X-ray derived redshift $z_X$ with the one obtained by means of optical spectroscopy $z_o$. We explore how a number of priors on metallicity and luminosity can be effectively used to reduce catastrophic errors. The $\Delta C_{stat}$ provides the most efficient means for discarding wrong redshift measures and to estimate the actual error on $z_X$. We identify a simple and efficient procedure for optimally measuring the redshifts from the X-ray spectral analysis of clusters of galaxies. When this procedure is applied to mock catalogs extracted from high sensitivity, wide-area cluster surveys, such as those proposed with Wide Field X-ray Telescope (WFXT) mission, it is possible to obtain a complete samples of X-ray clusters with reliable redshift measurements, thus avoiding time-consuming optical spectroscopic observations. This methodology will make it possible to trace cosmic growth by studying the evolution of the cluster mass function directly using X-ray data.

The cool-core bias in X-ray galaxy cluster samples

When selecting flux-limited cluster samples, the detection efficiency of X-ray instruments is not the same for centrally-peaked and flat objects, which introduces a bias in flux-limited cluster samples. We quantify this effect in the case of a well-known cluster sample, HIFLUGCS. We simulate a population of X-ray clusters with various surface-brightness profiles, and use the instrumental characteristics of the ROSAT All-Sky Survey (RASS) to select flux-limited samples similar to the HIFLUGCS sample and predict the expected bias. For comparison, we also estimate observationally the bias in the HIFLUGCS sample using XMM-Newton and ROSAT data. We find that the selection of X-ray cluster samples is significantly biased ($\sim29%$) in favor of the peaked, Cool-Core (CC) objects, with respect to Non-Cool-Core (NCC) systems. Interestingly, we find that the bias affects the low-mass, nearby objects (groups, poor clusters) much more than the more luminous objects (i.e massive clusters). We also note a moderate increase of the bias for the more distant systems. Observationally, we propose to select the objects according to their flux in a well-defined physical range excluding the cores, $0.2r_{500}-r_{500}$, to get rid of the bias. From the fluxes in this range, we reject 13 clusters out of the 64 in the HIFLUGCS sample, none of which appears to be NCC. As a result, we estimate that less than half (35-37%) of the galaxy clusters in the local Universe are strong CC. In the paradigm where the CC objects trace relaxed clusters as opposed to unrelaxed, merging objects, this implies that to the present day the majority of the objects are not in a relaxed state. From this result, we estimate a rate of heating events of $\sim1/3$ Gyr$^{-1}$ per dark-matter halo.

SIMULATIONS OF THE SUNYAEV-ZEL'DOVICH POWER SPECTRUM WITH ACTIVE GALACTIC NUCLEUS FEEDBACK

We explore how radiative cooling, supernova feedback, cosmic rays and a new model of the energetic feedback from active galactic nuclei (AGN) affect the thermal and kinetic Sunyaev-Zel'dovich (SZ) power spectra. To do this, we use a suite of hydrodynamical TreePM-SPH simulations of the cosmic web in large periodic boxes and tailored higher resolution simulations of individual galaxy clusters. Our AGN feedback simulations match the recent universal pressure profile and cluster mass scaling relations of the REXCESS X-ray cluster sample better than previous analytical or numerical approaches. For multipoles $\ell\lesssim 2000$, our power spectra with and without enhanced feedback are similar, suggesting theoretical uncertainties over that range are relatively small, although current analytic and semi-analytic approaches overestimate this SZ power. We find the power at high 2000-10000 multipoles which ACT and SPT probe is sensitive to the feedback prescription, hence can constrain the theory of intracluster gas, in particular for the highly uncertain redshifts $>0.8$. The apparent tension between $\sigma_8$ from primary cosmic microwave background power and from analytic SZ spectra inferred using ACT and SPT data is lessened with our AGN feedback spectra.

Simulating the γ-ray emission from galaxy clusters: a universal cosmic ray spectrum and spatial distribution

Entering a new era of high-energy gamma-ray experiments, there is an exciting quest for the first detection of gamma-ray emission from clusters of galaxies. To complement these observational efforts, we use high-resolution simulations of a broad sample of galaxy clusters, and follow self-consistent cosmic ray (CR) physics using an improved spectral description. We study CR proton spectra as well as the different contributions of the pion decay and inverse Compton emission to the total flux and present spectral index maps. We find a universal spectrum of the CR component in clusters with surprisingly little scatter across our cluster sample. The spatial CR distribution also shows approximate universality; it depends however on the cluster mass. This enables us to derive a semi-analytic model for both, the distribution of CRs as well as the pion-decay gamma-ray emission that results from hadronic CR interactions with ambient gas protons. In addition, we provide an analytic framework for the inverse Compton emission that is produced by shock-accelerated CR electrons and valid in the full gamma-ray energy range. Combining the complete sample of the brightest X-ray clusters observed by ROSAT with our gamma-ray scaling relations, we identify the brightest clusters for the gamma-ray space telescope Fermi and current imaging air Cherenkov telescopes (MAGIC, HESS, VERITAS). We reproduce the result in Pfrommer (2008), but provide somewhat more conservative predictions for the fluxes in the energy regimes of Fermi and imaging air Cherenkov telescopes when accounting for the bias of `artificial galaxies' in cosmological simulations. We find that it will be challenging to detect cluster gamma-ray emission with Fermi after the second year but this mission has the potential of constraining interesting values of the shock acceleration efficiency after several years of surveying.

Simulating cosmic rays in clusters of galaxies – III. Non-thermal scaling relations and comparison to observations

Complementary views of galaxy clusters in the radio synchrotron, hard X-ray inverse Compton and high-energy γ-ray regimes are critical in calibrating them as high-precision cosmological probes. We present predictions for scaling relations between cluster mass and these non-thermal observables. To this end, we use high-resolution simulations of a sample of galaxy clusters spanning a mass range of almost two orders of magnitudes, and follow self-consistent cosmic ray physics on top of the radiative hydrodynamics. We model relativistic electrons that are accelerated at cosmological structure formation shocks and those that are produced in hadronic interactions of cosmic rays with ambient gas protons. Calibrating the magnetic fields of our model with Faraday rotation measurements, the synchrotron emission of our relativistic electron populations matches the radio synchrotron luminosities and morphologies of observed giant radio haloes and minihaloes surprisingly well. Using the complete sample of the brightest X-ray clusters observed by ROSAT in combination with our γ-ray scaling relation, we predict GLAST to detect about ten clusters allowing for Eddington bias due to the scatter in the scaling relation. The expected brightest γ-ray clusters are Ophiuchus, Fornax, Coma, A3627, Perseus and Centaurus. The high-energy γ-ray emission above 100 MeV is dominated by pion decays resulting from hadronic cosmic ray interactions. We provide an absolute lower flux limit for the γ-ray emission of Coma in the hadronic model which can be made tighter for magnetic field values derived from rotation measurements to match the GLAST sensitivity, providing thus a unique test for the possible hadronic origin of radio haloes. Our predicted hard X-ray emission, due to inverse Compton emission of shock accelerated and hadronically produced relativistic electrons, falls short of the detections in Coma and Perseus by a factor of 50. This casts doubts on inverse Compton interpretation and reinforces the known discrepancy of magnetic field estimates from Faraday rotation measurements and those obtained by combining synchrotron and inverse Compton emission.

An Increase in the Faint Red Galaxy Population in Massive Clusters sincez∼ 0.5

We compare the luminosity functions for red galaxies lying on the restframe (U-V) color-magnitude sequence in a homogeneous sample of ten X-ray luminous clusters from the MACS survey at z~0.5 to a similarly selected X-ray cluster sample at z~0.1. We exploit deep Hubble Space Telescope ACS imaging in the F555W and F814W passbands of the central 1.2Mpc diameter regions of the distant clusters to measure precise colors for the galaxies in these regions and statistically correct for contamination by field galaxies using observations of blank fields. We apply an identical analysis to ground-based photometry of the z~0.1 sample. This comparison demonstrates that the number of faint, Mv~ -19, red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyrs between z~0.5 and z~0.1. We quantify this difference by measuring the dwarf to giant ratio on the red sequence which increases by a factor of at least 2.2+/- 0.4 since z~0.5. This is consistent with the idea that many faint, blue star-forming galaxies in high density environments are transforming onto the red sequence in the last half of the Hubble time.

TheXMMLarge-Scale Structure survey: a well-controlled X-ray cluster sample over the D1 CFHTLS area

We present the XMM Large-Scale Structure Survey (XMM-LSS) cluster catalogue corresponding to the Canada-France-Hawaii Telescope Legacy Survey Dl area. The list contains 13 spectroscopically confirmed, X-ray selected galaxy clusters over 0.8 deg 2 to a redshift of unity and so constitutes the highest density sample of clusters to date. Cluster X-ray bolometric luminosities range from 0.03 to 5 x 10 44 erg s -1 . In this study, we describe our catalogue construction procedure: from the detection of X-ray cluster candidates to the compilation of a spectroscopically confirmed cluster sample with an explicit selection function. The procedure further provides basic X-ray products such as cluster temperature, flux and luminosity. We detected slightly more clusters with (0.5-2.0 keV) X-ray fluxes of >2 x 10 -14 erg s -1 cm -2 than we expected based on expectations from deep ROSAT surveys. We also present the luminosity-temperature relation for our nine brightest objects possessing a reliable temperature determination. The slope is in good agreement with the local relation, yet compatible with a luminosity enhancement for the 0.15 < z < 0.35 objects having 1 < T < 2 keV, a population that the XMM-LSS is identifying systematically for the first time. The present study permits the compilation of cluster samples from XMM images whose selection biases are understood. This allows, in addition to studies of large-scale structure, the systematic investigation of cluster scaling law evolution, especially for low mass X-ray groups which constitute the bulk of our observed cluster population. All cluster ancillary data (images, profiles, spectra) are made available in electronic form via the XMM-LSS cluster data base.

An improved deprojection and PSF-deconvolution technique for galaxy-cluster X-ray surface-brightness profiles

We have developed a regularisation procedure for the direct deprojection and PSF-deconvolution of X-ray surface brightness profiles of clusters of galaxies. This procedure allows us to obtain accurate density profiles in a straightforward manner from X-ray observations (in particular data from XMM-Newton, where the PSF correction is important), while retaining information about substructure in the gas distribution, in contrast to analytic modelling of the profiles. In addition to describing our procedure, we present here a detailed investigation of the accuracy of the method and its error calculations over a wide range of input profile characteristics and data quality using Monte Carlo simulations. We also make comparisons with gas density profiles obtained from Chandra observations, where the PSF correction is small, and with profiles obtained using analytic modelling, which demonstrate that our procedure is a useful improvement over standard techniques. This type of method will be especially valuable in the ongoing analysis of unbiased and complete samples of X-ray clusters, both local and distant, helping to improve the quality of their results.

The galaxy cluster X-ray luminosity–gravitational mass relation in the light of the WMAP 3rd year data

Context.The 3rd year WMAP results mark a shift in best fit values of cosmological parameters compared to the 1st year data and the concordance cosmological model.

Systematic Bias in an Estimate of the Cluster Mass and the Fluctuation Amplitude from Cluster Abundance Statistics

Abstract We revisit an estimate of the mass fluctuation amplitude, $\sigma_{8}$, from the observational X-ray cluster abundance. In particular, we examine the effect of the systematic difference between the cluster virial mass estimated from the X-ray spectroscopy, $M_\mathrm{vir,\, spec}$, and the true virial mass of the corresponding halo, $M_\mathrm{vir}$. Mazzotta et al. (2004, MNRAS, 354, 10) recently pointed out the possibility that $\alpha_{M} = M_\mathrm{vir,\, spec} / M_\mathrm{vir}$ is systematically lower than unity. We performed a statistical analysis by combining the latest X-ray cluster sample and the improved theoretical models, and found that $\sigma_8 \sim 0.76 \pm 0.01 + 0.50 \, (1-\alpha_{M})$ for $0.5 \le \alpha_{M} \le 1$, where the quoted errors are only statistical. Thus, if $\alpha_{M} \sim 0.7$, the value of $\sigma_8$ from cluster abundance alone is now in better agreement with other cosmological data, including the cosmic microwave background, the galaxy power spectrum and the weak lensing data. The current study also illustrates the importance of possible systematic effects in mapping real clusters to underlying dark halos, which changes the interpretation of cluster abundance statistics.

Erratum: The XMM Large-Scale Structure survey: an initial sample of galaxy groups and clusters to a redshift z &lt; 0.6

We present X-ray and optical spectroscopic observations of twelve galaxy groups and clusters identified within the XMM Large-Scale Structure (LSS) survey. Groups and clusters are selected as extended X-ray sources from a 3.5 deg2 XMM image mosaic above a flux limit 8e-15 ergs/s/cm2 in the [0.5-2] keV energy band. Deep BVRI images and multi-object spectroscopy confirm each source as a galaxy concentration located within the redshift interval 0.29<z<0.56. We combine line-of-sight velocity dispersions with the X-ray properties of each structure computed from a two-dimensional surface brightness model and a single temperature fit to the XMM spectral data. The resulting distribution of X-ray luminosity, temperature and velocity dispersion indicate that the XMM-LSS survey is detecting low-mass clusters and galaxy groups to redshifts z < 0.6. Confirmed systems display little or no evidence for X-ray luminosity evolution at a given X-ray temperature compared to lower redshift X-ray group and cluster samples. A more complete understanding of these trends will be possible with the compilation of a statistically complete sample of galaxy groups and clusters anticipated within the continuing XMM-LSS survey.

The XMM Large-Scale Structure survey: an initial sample of galaxy groups and clusters to a redshift z &lt; 0.6

We present X-ray and optical spectroscopic observations of 12 galaxy groups and clusters identified within the X-ray Multi-Mirror (XMM) Large-Scale Structure (LSS) survey. Groups and clusters are selected as extended X-ray sources from a 3.5 deg 2 XMM image mosaic above a flux limit 8 × 10 −15 erg s −1 cm −2 in the [0.5‐2] keV energy band. Deep BVRI images and multi-object spectroscopy confirm each source as a galaxy concentration located within the redshift interval 0.29 < z < 0.56. We combine line-of-sight velocity dispersions with the X-ray properties of each structure computed from a two-dimensional surface brightness model and a single temperature fit to the XMM spectral data. The resulting distribution of X-ray luminosity, temperature and velocity dispersion indicate that the XMM‐LSS survey is detecting low-mass clusters and galaxy groups to redshifts z < 0.6. Confirmed systems display little or no evidence for X-ray luminosity evolution at a given X-ray temperature compared to lower-redshift X-ray group and cluster samples. A more complete understanding of these trends will be possible with the compilation of a statistically complete sample of galaxy groups and clusters anticipated within the continuing XMM‐LSS survey. Ke yw ords: surveys ‐ large-scale structure of Universe ‐ X-rays: galaxies: clusters.

A CFH12k lensing survey of X-ray luminous galaxy clusters

We describe the weak lensing methodology we have applied to multi-colour CFH12k imaging of a homogeneously- selected sample of luminous X-ray clusters at z ∼ 0.2. The aim of our survey is to understand the variation in cluster structure and dark matter profile within rich clusters. The method we describe converts a fully reduced CFH12k image into constraints on the cluster mass distribution in two steps: (1) determination of the true shape of faint (lensed) galaxies, including object detection, point spread function (PSF) determination, galaxy shape measurement with errors; (2) conversion of the faint galaxy catalogue into reliable mass constraints using a range of 1D and 2D lensing techniques. Mass estimates are derived indepen- dently from each of the three images taken in separate filters to quantify the systematic uncertainties. Finally, we compare the cluster mass model to the light distribution of cluster members as derived from our imaging data. To illustrate the method, we apply it to the well-studied cluster Abell 1689 (z = 0.184). In this cluster, we detect the gravitational shear signal out to ∼ 3M pc at >3-σ significance. The two-dimensional mass reconstruction has a ∼10-σ significance peak centered on the brightest clus- ter galaxy. The weak lensing profile is well fitted by a NFW mass profile with M200 = 14.1 +6.3 −4.7 × 10 14 M

H0from an orientation-unbiased sample of Sunyaev-Zel'dovich and X-ray clusters

We have observed the Sunyaev-Zel'dovich (SZ) effect in a sample of five moderate-redshift clusters with the Ryle Telescope, and used them in conjunction with X-ray imaging and spectral data from ROSAT and ASCA to measure the Hubble constant. This sample was chosen with a strict X-ray flux limit using both the Bright Cluster Sample and the Northern ROSAT All-Sky Survey (RASS) cluster catalogues to be well above the surface brightness limit of the RASS, and hence to be unbiased with respect to the orientation of the cluster. This controls a major potential systematic effect in the SZ/X-ray method of measuring H 0 . Taking the weighted geometric mean of the results and including the main sources of error, namely the noise in the SZ measurement, the uncertainty in the X-ray temperatures and the unknown ellipticity and substructure of the clusters, we find H 0 = 59 +10 -9 (random) +8 -7 (systematic) km s -1 Mpc -1 assuming a standard cold dark matter model with Ω M = 1.0, Ω A = 0.0 or H 0 = 66 +11 +9 -10 -8 km s -1 Mpc -1 if Ω M = 0.3, Ω A = 0.7.

The REFLEX galaxy cluster survey

For the first time the large-scale clustering and the mean abundance of galaxy clusters are analysed simultaneously to get precise constraints on the normalized cosmic matter density and the linear theory RMS fluctuations in mass . A self-consistent likelihood analysis is described which combines, in a natural and optimal manner, a battery of sensitive cosmological tests where observational data are represented by the (Karhunen-Loéve) eigenvectors of the sample correlation matrix. This method breaks the degeneracy between and . The cosmological tests are performed with the ROSAT ESO Flux-Limited X-ray (REFLEX) cluster sample. The computations assume cosmologically flat geometries and a non-evolving cluster population mainly over the redshift range . The REFLEX sample gives the cosmological constraints and their random errors of and . Possible systematic errors are evaluated by estimating the effects of uncertainties in the value of the Hubble constant, the baryon density, the spectral slope of the initial scalar fluctuations, the mass/X-ray luminosity relation and its intrinsic scatter, the biasing scheme, and the cluster mass density profile. All these contributions sum up to total systematic errors of and .

The Evolution of Brightest Cluster Galaxies

In this contribution we review the properties of Brightest Cluster Galaxies (BCGs) and discuss the impact that X-ray cluster selection is having on their use as cosmological probes. BCGs form a unique galaxy population. They are located near the gravitational centre of galaxy clusters and are the most massive galaxies in the universe, being some 10 times more luminous than L* systems. Historically, BCGs have been credited with small intrinsic dispersion in their absolute magnitudes (Δ≃0.2–0.3 mag) and used as standard candles to constrain the cosmological parameters. Although indirect signs of mass accretion out to z≃1 have been observed, uncovering their full evolutionary picture has remained an elusive goal. Studies of BCGs based on serendipitiously discovered X-ray cluster samples, particularly from ROSAT, provide large numbers of unbiased clusters at z≤1. Furthermore X-ray emission guarantees the presence of a large gravitationally bound potential well and the X-ray information can be used to locate the centroids of clusters, aiding the identification of the BCG. We show that this has important consequences for studies of distance determination and large-scale streaming flows based on the optical properties of BCGs. Recent results based on X-ray selected clusters show large differences in near-IR BCG properties with their cluster environment; such that those in clusters with Lx≥1.9×1044erg s-1 are brighter and more uniform than those in their low-Lx counterparts. The BCGs in highLx systems show no evidence of having undergone mass growth, whereas those in low Lx systems show a widerrange of evolution, with evidence that some have grown by a factor of 4 ormore since z≃1. These results are a direct indication of howa single homogeneous population of galaxies evolves and are a challenge to simple semi-analytical hierarchical models. If future observations at high redshift are to seriously challenge theory then better predictions of the evolutionary process are required.

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).