Observational Constraints on General Relativistic Energy Conditions, Cosmic Matter Density and Dark Energy from X-Ray Clusters of Galaxies and Type-Ia Supernovae

New observational constraints on the cosmic matter density $\Omega_m$ and an effectively redshift-independent equation of state parameter $w_x$ of the dark energy are obtained while simultaneously testing the strong and null energy conditions of general relativity on macroscopic scales. The combination of REFLEX X-ray cluster and type-Ia supernova data shows that for a flat Universe the strong energy condition might presently be violated whereas the null energy condition seems to be fulfilled. This provides another observational argument for the present accelerated cosmic expansion and the absence of exotic physical phenomena related to a broken null energy condition. The marginalization of the likelihood distributions is performed in a manner to include a large fraction of the recently discussed possible systematic errors involved in the application of X-ray clusters as cosmological probes. This yields for a flat Universe, $\Omega_m=0.29^{+0.08}_{-0.12}$ and $w_x=-0.95^{+0.30}_{-0.35}$ ($1\sigma$ errors without cosmic variance). The scatter in the different analyses indicates a quite robust result around $w_x=-1$, leaving little room for the introduction of new energy components described by quintessence-like models or phantom energy. The most natural interpretation of the data is a positive cosmological constant with $w_x=-1 or something like it.Comment: 11 pages, 5 figures, Astron. Astrophys. (in press

The 3D soft X-ray cluster-AGN cross-correlation function in the ROSAT NEP survey

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.Comment: 4 pages, 2 figure, proceedings of the Conference "At the edge of the Universe", Sintra Portugal, October 2006. To be published on the Astronomical Society of the Pacific Conference Series (ASPCS

Observational Constraints on General Relativistic Energy Conditions, Cosmic Matter Density and Dark Energy from X-Ray Clusters of Galaxies and Type-la Supernovae

New observational constraints on the cosmic matter density Ωm and an effectively redshift-independent equation of state parameter wx of the dark energy are obtained while simultaneously testing the strong and null energy conditions of general relativity on macroscopic scales. The combination of REFLEX X-ray cluster and type-Ia supernova data shows that for a flat Universe the strong energy condition might presently be violated whereas the null energy condition seems to be fulfilled. This provides another observational argument for the present accelerated cosmic expansion and the absence of exotic physical phenomena related to a broken null energy condition. The marginalization of the likelihood distributions is performed in a manner to include a large fraction of the recently discussed possible systematic errors involved in the application of X-ray clusters as cosmological probes. This yields for a flat Universe, Ωm = 0.29+0.08 and wx = −0.95+0.30 (1σ errors without −0.12 −0.35 cosmic variance). The scatter in the different analyses indicates a quite robust result around wx = −1, leaving little room for the introduction of new energy components described by quintessence-like models or phantom energy. The most natural interpretation of the data is a positive cosmological constant with wx = −1 or something like it

The Representative XMM-Newton Cluster Structure Survey (REXCESS) of an X-ray Luminosity Selected Galaxy Cluster Sample

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. To improve on this situation we have started an XMM-Newton Large Programme for the in-depth study of a representative sample of 33 galaxy clusters, selected in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV band. This paper introduces the sample, compiles properties of the clusters, and provides detailed information on the sample selection function. We describe the selection of a nearby galaxy cluster sample that makes optimal use of the XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity coverage for the full range from poor clusters to the most massive objects in the Universe. For the clusters in the sample, X-ray fluxes are derived and compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We find that the fluxes and the flux errors have been reliably determined in the ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample selection function documented in detail in this paper to determine the X-ray luminosity function, and compare it with the luminosity function of the entire REFLEX sample. We also discuss morphological peculiarities of some of the sample members. The sample and some of the background data given in this introductory paper will be important for the application of these data in the detailed studies of cluster structure, to appear in forthcoming publications.Comment: 17 pages, 17 figures; to appear in A&A. A pdf version with full-quality figures can be found at ftp://ftp.xray.mpe.mpg.de/people/gwp/xmmlp/xmmlp.pd

CHANDRA reveals galaxy cluster with the most massive nearby cooling core, RXCJ1504.1-0248

A CHANDRA follow-up observation of an X-ray luminous galaxy cluster with a compact appearance, RXCJ1504.1-0248 discovered in our REFLEX Cluster Survey, reveals an object with one of the most prominent cluster cooling cores. With a core radius of ~30 kpc smaller than the cooling radius with ~140 kpc more than 70% of the high X-ray luminosity of Lbol = 4.3 10e45 erg s-1 of this cluster is radiated inside the cooling radius. A simple modeling of the X-ray morphology of the cluster leads to a formal mass deposition rate within the classical cooling flow model of 1500 - 1900 Msun yr-1 (for h=0.7), and 2300 - 3000 Msun yr-1 (for h=0.5). The center of the cluster is marked by a giant elliptical galaxy which is also a known radio source. Thus it is very likely that we observe one of the interaction systems where the central cluster AGN is heating the cooling core region in a self-regulated way to prevent a massive cooling of the gas, similar to several such cases studied in detail in more nearby clusters. The interest raised by this system is then due to the high power recycled in RXCJ1504-0248 over cooling time scales which is about one order of magnitude higher than what occurs in the studied, nearby cooling core clusters. The cluster is also found to be very massive, with a global X-ray temperature of about 10.5 keV and a total mass of about 1.7 10e15 Msun inside 3 Mpc.Comment: accepted for publication in Astrophys. Journal, 10 figure

Probing Dark Energy with Baryonic Acoustic Oscillations at High Redshifts

The phases as well as the amplitudes of baryonic acoustic oscillations can be extracted out of a galaxy power spectrum and used as a standard ruler in a cosmological test. 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 (FITEX) 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 against a theoretical model template to derive constraints on the $w$ parameter. The phenomenological fitting function is found to model the various distortions of the galaxy power spectrum to the sub-percent 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.Comment: accepted for publication in Astron. Astrophys., 16 pages, 16 figure

Discovery of an X-ray-Luminous Galaxy Cluster at z=1.4

We report the discovery of a massive, X-ray-luminous cluster of galaxies at z=1.393, the most distant X-ray-selected cluster found to date. XMMU J2235.3-2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314. VLT-FORS2 R and z band snapshot imaging reveals an over-density of red galaxies in both angular and color spaces. The galaxy enhancement is coincident in the sky with the X-ray emission; the cluster red sequence at R-z ~ 2.1 identifies it as a high-redshift candidate. Subsequent VLT-FORS2 multi-object spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38<z<1.40. The preliminary cluster velocity dispersion is 762+/-265 km/s. VLT-ISAAC Ks and J band images underscore the rich distribution of red galaxies associated with the cluster. Based on a 45 ks XMM-Newton observation, we find the cluster has an aperture-corrected, unabsorbed X-ray flux of f_X = (3.6 +/- 0.3) x 10^{-14} erg/cm^2/s, a rest-frame X-ray luminosity of L_X = (3.0 +/- 0.2) x 10^{44} h_70^{-2} erg/s (0.5--2.0 keV), and a temperature of kT=6.0 (+2.5, -1.8) keV. Though XMMU J2235.3-2557 is likely the first confirmed z>1 cluster found with XMM-Newton, the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of z>1 clusters through the joint use of X-ray and large, ground-based telescopes.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letters, a high-resolution version is available at http://www.astro.lsa.umich.edu/~cmullis/papers/Mullis_et_al_2005a.pdf, additional information is available at http://www.astro.lsa.umich.edu/~cmullis/research/xmmuj223

Towards a Precision Cosmology from Starburst Galaxies at z>2

This work investigates the use of a well-known empirical correlation between the velocity dispersion, metallicity, and luminosity in H beta of nearby HII galaxies to measure the distances to HII-like starburst galaxies at high redshifts. This correlation is applied to a sample of 15 starburst galaxies with redshifts between z=2.17 and z=3.39 to constrain Omega_m, using data available from the literature. A best-fit value of Omega_m = 0.21 +0.30 -0.12 in a Lambda-dominated universe and of Omega_m = 0.11 +0.37 -0.19 in an open universe is obtained. A detailed analysis of systematic errors, their causes, and their effects on the values derived for the distance moduli and Omega_m is carried out. A discussion of how future work will improve constraints on Omega_m by reducing the errors is also presented.Comment: 7 pages, 3 figures, accepted for publication in MNRA

A direct limit on the turbulent velocity of the intracluster medium in the core of Abell 1835 from XMM-Newton

We examine deep XMM-Newton Reflection Grating Spectrometer (RGS) observations of the X-ray luminous galaxy cluster A1835. For the first time in a galaxy cluster we place direct limits on turbulent broadening of the emission lines. This is possible because the coolest X-ray emitting gas in the cluster, which is responsible for the lines, occupies a small region within the core. The most conservative determination of the 90 per cent upper limit on line-of-sight, non-thermal, velocity broadening is 274 km/s, measured from the emission lines originating within 30 kpc radius. The ratio of turbulent to thermal energy density in the core is therefore less than 13 per cent. There are no emission lines in the spectrum showing evidence for gas below ~3.5 keV. We examine the quantity of gas as a function of temperature and place a limit of 140 Msun/yr (90 per cent) for gas cooling radiatively below 3.85 keV.Comment: 5 pages, accepted by MNRAS, includes minor change suggested by refere