We present the final data from the spectroscopic survey of the ROSAT-ESO Flux-Limited X-ray (REFLEX) catalog of galaxy clusters. The REFLEX survey covers 4.24 steradians (34% of the entire sky) below a declination of 2.5 deg and at high Galactic latitude (|b| > 20 deg). The REFLEX catalog includes 447 entries with a median redshift of 0.08 and is better than 90% complete to a limiting flux fx = 3x10^{-12} erg s^{-1} cm^{-2} (0.1 to 2.4 keV), representing the largest statistically homogeneous sample of clusters drawn from the ROSAT All-Sky Survey (RASS) to date. Here we describe the details of the spectroscopic observations carried out at the ESO 1.5 m, 2.2 m, and 3.6 m telescopes, as well as the data reduction and redshift measurement techniques. The spectra typically cover the wavelength range 3600-7500 A at a FWHM resolution of ~14 A, and the measured redshifts have a total rms error of ~100 km s^{-1}. In total we present 1406 new galaxy redshifts in 192 clusters, most of which previously did not have any redshift measured. Finally, the luminosity/redshift distributions of the cluster sample and a comparison to the no-evolution expectations from the cluster X-ray luminosity function are presented.

We present the final data from the spectroscopic survey of the ROSAT-ESO Flux-Limited X-ray (REFLEX) catalog of galaxy clusters. The REFLEX survey covers 4.24 steradians (34% of the entire sky) below a declination of 2.5 deg and at high Galactic latitude (|b| > 20 deg). The REFLEX catalog includes 447 entries with a median redshift of 0.08 and is better than 90% complete to a limiting flux fx = 3x10^{-12} erg s^{-1} cm^{-2} (0.1 to 2.4 keV), representing the largest statistically homogeneous sample of clusters drawn from the ROSAT All-Sky Survey (RASS) to date. Here we describe the details of the spectroscopic observations carried out at the ESO 1.5 m, 2.2 m, and 3.6 m telescopes, as well as the data reduction and redshift measurement techniques. The spectra typically cover the wavelength range 3600-7500 A at a FWHM resolution of ~14 A, and the measured redshifts have a total rms error of ~100 km s^{-1}. In total we present 1406 new galaxy redshifts in 192 clusters, most of which previously did not have any redshift measured. Finally, the luminosity/redshift distributions of the cluster sample and a comparison to the no-evolution expectations from the cluster X-ray luminosity function are presented.

For the spectral determination of the temperature distribution in galaxy clusters we developed a double background subtraction method, and applied it to 2 morphology unbiased samples of X-ray luminous galaxy clusters (12 at z ∼0.2, 13 at z ∼0.3) observed by XMM-Newton. The scaled profiles of the X-ray properties show a self-similar behaviour above 0.2r500 for both samples. The tight X-ray scaling relations of the 2 samples show no evolution compared to the nearby and more distant samples. The X-ray lensing masses converge for most clusters.

Context.The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass.

Local scaling properties of the co-added foreground-cleaned three-year Wilkinson Microwave Anisotropy Probe (WMAP) data are estimated using weighted scaling indices alpha. The scaling index method is - for the first time - adapted and applied to the case of spherical symmetric spatial data. The results are compared with 1000 Monte Carlo simulations based on Gaussian fluctuations with a best-fitting Lambda cold dark matter power spectrum and WMAP-like beam and noise properties. Statistical quantities based on the scaling indices, namely the mean and standard deviation, a combination of both and probability-based measures are determined. We find for most of the test statistics highly significant deviations from the Gaussian hypothesis. Using a very conservative chi(2) statistics, which averages over all scales, we detect non-Gaussianity for the full sky at a level of 97.3 per cent regarding the mean, 97.3 per cent for the standard deviation and 97.7 per cent for a combination of mean and standard deviation. For the Northern hemisphere, the signatures of non-Gaussianities are more pronounced and we obtain 97.7 per cent (mean), 99.8 per cent (standard deviation) and 99.0 per cent (combination), whereas the Southern hemisphere is more consistent with Gaussianity [94.8 per cent (mean), 71.0 per cent (standard deviation) and 91.5 per cent (combination)]. These differences between Northern and Southern hemispheres induce pronounced asymmetries, which can be interpreted as a global lack of structure in the Northern hemisphere, which is consistent with previous findings. Furthermore, we detect a localized anomaly in the Southern hemisphere giving rise to highly significant signature for non-Gaussianity in the spectrum of scaling indices P(alpha). We identify this signature as the cold spot, which was also already detected in the first-year WMAP data. Our results provide further evidence for both the presence of non-Gaussianities and the presence of asymmetries in the WMAP three-year data. More detailed band- and year-wise analyses are needed to elucidate the origin of the detected anomalies. In either case the scaling indices provide powerful non-linear statistics to analyse cosmic microwave background maps.

Statistical studies of ROSAT selected cluster samples at z 1. We are currently engaged in the XMM-Newton Distant Cluster Project (XDCP) [1], a serendipitous X-ray survey with the goal to construct a statistically complete sample of at least 30 galaxy clusters at z>1 for evolutionary and cosmological studies. In these proceedings we show that the prospects of finding very distant cooling core clusters are very promising for ongoing XMM surveys if these systems exist at lookback times of more than 8 Gyrs.

X-ray surveys facilitate investigations of the environment of AGNs. Deep Chandra observations revealed that the AGNs source surface density rises near clusters of galaxies. The natural extension of these works is the measurement of spatial clustering of AGNs around clusters and the investigation of relative biasing between active galactic nuclei and galaxies near clusters.The major aims of this work are to obtain a measurement of the correlation length of AGNs around clusters and a measure of the averaged clustering properties of a complete sample of AGNs in dense environments. We present the first measurement of the soft X-ray cluster-AGN cross-correlation function in redshift space using the data of the ROSAT-NEP survey. The survey covers 9x9 deg^2 around the North Ecliptic Pole where 442 X-ray sources were detected and almost completely spectroscopically identified. We detected a >3sigma significant clustering signal on scales s<50 h70^-1 Mpc. We performed a classical maximum-likelihood power-law fit to the data and obtained a correlation length s_0=8.7+1.2-0.3 h_70-1 Mpc and a slope gamma=1.7$^+0.2_-0.7 (1sigma errors). This is a strong evidence that AGNs are good tracers of the large scale structure of the Universe. Our data were compared to the results obtained by cross-correlating X-ray clusters and galaxies. We observe, with a large uncertainty, that the bias factor of AGN is similar to that of galaxies.

Context. X-ray surveys facilitate investigations of the environment of AGNs. Deep Chandra observations have revealed that the AGN source surface density rises near clusters of galaxies. The natural extension of this finding is the measurement of spatial clustering of AGNs around clusters and the investigation of relative biasing between active galactic nuclei and galaxies near clusters. Aims. We aim to measure the correlation length of AGNs around clusters and the average clustering properties of a complete sample of AGNs in a dense environment. Methods. We present the first measurement of the soft X-ray cluster-AGN cross-correlation function in redshift space using the data of the ROSAT-NEP survey. The survey covers 9 x 9 deg 2 around the North Ecliptic Pole where 442 X-ray sources were detected and almost completely spectroscopically identified. Results. We detected a >3σ significant clustering signal on scales s < 50 h -1 70 Mpc. We performed a classical maximum-likelihood power-law fit to the data and obtained a correlation length so = 8.7 +1.2 -0.3 h -1 70 Mpc and a slope y = 1.7 +0.2 -0.7 (1σ errors). Conclusions. This is strong evidence that AGNs are good tracers of the large scale structure of the Universe. Our data were compared to the results obtained by cross-correlating X-ray clusters and galaxies. We observe, with a large uncertainty, a similar behaviour of AGN clustering around clusters similar to the clustering of galaxies around clusters.

Context. The phases and amplitudes of baryonic acoustic oscillations in the galaxy power spectrum can be used as cosmological probes to constrain different cosmological models and especially the equation of state parameter w of dark energy. Aims. It will be shown that the phases as well as the amplitudes of baryonic acoustic oscillations can be extracted out of a galaxy power spectrum, including various observational effects like growth suppression, redshift space distortions, and galaxy biasing. The phases of baryonic acoustic oscillations are used as a standard ruler for a cosmological test to constrain w over a large redshift range with a minimum of assumptions. Methods. A non-oscillating phenomenological fitting function is used to extract the oscillatory part of the galaxy power spectrum and to disentangle phase information from amplitude information. The method is tested with simulated data of the Hubble Volume Simulation to include redshift space effects, non-linear structure growth, and biasing. A cosmological test is introduced, which compares the extracted oscillations to a theoretical model template to derive constraints on the w parameter. Results. The phenomenological fitting function is found to model the various distortions of the galaxy power spectrum to the subpercent level. The various distortions only boost the amplitude of the oscillations. The theoretical template is accurate enough to test for small deviations in the phases of the oscillations, resulting from different w values. A cosmological test, using the baryonic acoustic oscillations as a standard ruler, is able to constrain w in a robust way. These constraints are based solely on the phase of these oscillations, assuming no errors of the value at the end of the Compton drag epoch, as the standard ruler. The w constraints are expected to further improve when the full two dimensional redshift space effects and the amplitudes of the baryonic acoustic oscillations are taken into account.

Using XMM-Newton, we observed 14 distant X-ray luminous (z similar to 0.3) galaxy clusters selected from the REFLEX survey (REFLEX-DXL sample). We derived the X-ray properties of the REFLEX-DXL galaxy clusters using a double background subtraction method in Zhang et al. (2004). Cluster mass measurements based on the X-ray data have been used to study the X-ray galaxy cluster scaling relations and their intrinsic scatter. This is important for the use of clusters of galaxies as cosmological probes. We found that the X-ray properties of the REFLEX-DXL sample show an approximate self-similar behavior above 0.15-0.2 virial radii. This helps us to establish tight cluster mass-observables scaling relations, in particular the M-T relation with a scatter of 0.3 for M.

We selected an unbiased, flux-limited and almost volume-complete sample of 13 distant, X-ray luminous (DXL, z similar to 0.3) clusters and one supplementary cluster at z = 0.2578 from the REFLEX Survey (the REFLEX-DXL sample). We performed a detailed study to explore their X-ray properties using XMM-Newton observations. Based on the precise radial distributions of the gas density and temperature, we obtained robust cluster masses and gas mass fractions. The average gas mass fraction of the REFLEX-DXL sample at r(500), 0.116 +/- 0.007, agrees with the previous cluster studies and the WMAP baryon fraction measurement. The scaled profiles of the surface brightness, temperature, entropy, gas mass and total mass are characterized by a self-similar behaviour at radii above 0.2-0.3 r(500). The REFLEX-DXL sample confirms the previous studies of the normalization of the scaling relations (L-T, L-M, M-T and M-gas-T) when the redshift evolution of the scaling relations is accounted for. We investigated the scatter of the scaling relations of the REFLEX-DXL sample. This gives the correlative scatter of (0.20, 0.10) for variable of (M, T) of the M-500-T relation, for example.

Understanding the dynamical structure and matter content of galaxy clusters is crucial for many cosmological and astrophysical applications. While optical studies provide information on the distribution and dynamics of the galaxies, allowing for a tentative reconstruction of the cluster mass distribution, X-ray observations provide complementary details through the study of the hot, X-ray luminous intracluster plasma which is confined by the cluster’s gravitational potential well. To exploit the avantage of such a combined approach we have been conducting observations with VIMOS at the ESO-VLT, the Wide Field Imager at the 2.2-m MPG/ ESO telescope at La Silla, and ESA’s XMM-Newton X-ray observatory. In this article we illustrate the power of the combination of these instruments for galaxy cluster studies.

PI: Will Sutherland, Institute of Astronomy, Cambridge, UK Ralf Bender, Hans Böhringer, Malcolm Bremer, Massimo Capaccioli, Lee Clewley, Gavin Dalton, Simon Driver, Alastair Edge, Jim Emerson, Thomas Erben, Philippe Heraudeau, Ulrich Hopp, Matt Jarvis, Konrad Kuijken, Jochen Liske, Richard McMahon, Yannick Mellier, Mark Neeser, John Peacock, Chris Pearson, Steve Phillipps, Mario Radovich, Kathy Romer, Roberto Saglia, Peter Schneider, Peter Schuecker, Stella Seitz, Roberto Silvotti, Andy Taylor, Edwin Valentijn, Bram Venemans.

We present a satellite mission concept to measure the dark energy equation of state parameter w with percent-level precision. The Very Ambitious Dark Energy Research satellite (VADER) is a multi-wavelength survey mission joining X-ray, optical, and IR instruments for a simultaneous spectral coverage from 4microns (0.3eV) to 10keV over a field of view (FoV) of 1 square degree. VADER combines several clean methods for dark energy studies, the baryonic acoustic oscillations in the galaxy and galaxy cluster power spectrum and weak lensing, for a joint analysis over an unrivalled survey volume. The payload consists of two XMM-like X-ray telescopes with an effective area of 2,800cm^2 at 1.5keV and state-of-the-art wide field DEPFET pixel detectors (0.1-10keV) in a curved focal plane configuration to extend the FoV. The X-ray telescopes are complemented by a 1.5m optical/IR telescope with 8 instruments for simultaneous coverage of the same FoV from 0.3 to 4 microns. The 8 dichroic-separated bands (u,g,r,z,J,H,K,L) provide accurate photometric galaxy redshifts, whereas the diffraction-limited resolution of the central z-band allows precise shape measurements for cosmic shear analysis. The 5 year VADER survey will cover a contiguous sky area of 3,500 square degrees to a depth of z~2 and will yield accurate photometric redshifts and multi-wavelength object parameters for about 175,000 galaxy clusters, one billion galaxies, and 5 million AGN. VADER will not only provide unprecedented constraints on the nature of dark energy, but will additionally extend and trigger a multitude of cosmic evolution studies to very large (>10 Gyrs) look-back times.

We investigate the relationship between soft X-ray luminosity and mass for low-redshift clusters of galaxies by comparing observed number counts and scaling laws to halo-based expectations of ΛCDM cosmologies. We model the conditional likelihood of halo luminosity as a lognormal distribution of fixed width, centered on a scaling relation, L ∝ Mpρ(z), and consider two values for s, appropriate for self-similar evolution or no evolution. Convolving with the halo mass function, we compute expected counts in redshift and flux that, after appropriate survey effects are included, we compare to REFLEX survey data. Counts alone provide only an upper limit on the scatter in mass at fixed luminosity, σln M < 0.4. We argue that the observed, intrinsic variance in the temperature-luminosity relation is directly indicative of mass-luminosity variance and derive σln M = 0.43 ± 0.06 from HIFLUGCS data. When added to the likelihood analysis, we derive values p = 1.59 ± 0.05, ln L15,0 = 1.34 ± 0.09, and σln M = 0.37 ± 0.05 for self-similar redshift evolution in a concordance (Ωm = 0.3, ΩΛ = 0.7, σ8 = 0.9) universe. The present-epoch intercept is sensitive to power spectrum normalization, L15,0 ∝ σ, and the slope is weakly sensitive to the matter density, p ∝ Ω. We find a substantially (factor 2) dimmer intercept and slightly steeper slope than the values published using hydrostatic mass estimates of the HIFLUGCS sample and show that a Malmquist bias of the X-ray flux-limited sample accounts for this effect. In light of new WMAP constraints, we discuss the interplay between parameters and sources of systematic error and offer a compromise model with Ωm = 0.24, σ8 = 0.85, and somewhat lower scatter σln M = 0.25, in which hydrostatic mass estimates remain accurate to ~15%. We stress the need for independent calibration of the L-M relation via weak gravitational lensing.

We report on an ongoing search for distant clusters of galaxies in archival XMM-Newton observations. The aim of the survey is to establish a sample of similar to 30 clusters at redshifts z > 1. Here we describe the strategy of the survey from X-ray source detection to optical follow up observations of distant cluster candidates. First results of a pilot survey and the discovery of the most distant X-ray selected cluster at z=1.4 are presented.