Wide Angle ROSAT Pointed Survey Research Articles

The WARPS Survey – VIII. Evolution of the galaxy cluster X-ray Luminosity Function

We present measurements of the galaxy cluster X-ray Luminosity Function (XLF) from the Wide Angle ROSAT Pointed Survey (WARPS) and quantify its evolution. WARPS is a serendipitous survey of the central region of ROSAT pointed observations and was carried out in two phases (WARPS-I and WARPS-II). The results here are based on a final sample of 124 clusters, complete above a flux limit of 6.5 10E-15 erg/s/cm2, with members out to redshift z ~ 1.05, and a sky coverage of 70.9 deg2. We find significant evidence for negative evolution of the XLF, which complements the majority of X-ray cluster surveys. To quantify the suggested evolution, we perform a maximum likelihood analysis and conclude that the evolution is driven by a decreasing number density of high luminosity clusters with redshift, while the bulk of the cluster population remains nearly unchanged out to redshift z ~ 1.1, as expected in a low density Universe. The results are found to be insensitive to a variety of sources of systematic uncertainty that affect the measurement of the XLF and determination of the survey selection function. We perform a Bayesian analysis of the XLF to fully account for uncertainties in the local XLF on the measured evolution, and find that the detected evolution remains significant at the 95% level. We observe a significant excess of clusters in the WARPS at 0.1 < z < 0.3 and LX ~ 2 10E42 erg/s compared with the reference low-redshift XLF, or our Bayesian fit to the WARPS data. We find that the excess cannot be explained by sample variance, or Eddington bias, and is unlikely to be due to problems with the survey selection function.

The WARPS Survey. VII. The WARPS‐II Cluster Catalog

We present the galaxy cluster catalog from the second, larger phase of the Wide Angle ROSAT Pointed Survey (WARPS), an X-ray selected survey for high-redshift galaxy clusters. WARPS is among the largest deep X-ray cluster surveys and is being used to study the properties and evolution of galaxy clusters. The WARPS-II sample contains 125 clusters serendipitously detected in a survey of 301 ROSAT PSPC pointed observations and covers a sky area of 56.7 deg2. Of these 125 clusters, 53 have not been previously reported in the literature. We have nearly complete spectroscopic follow-up of the clusters, which range in redshift from -->z = 0.029 to -->z = 0.92 with a median redshift of -->z = 0.29 and find 59 clusters with -->z ? 0.3 (29 not previously reported in the literature) and 11 clusters with -->z ? 0.6 (6 not previously reported). We also define a statistically complete subsample of 102 clusters above a uniform flux limit of -->6.5 ? 10?14 ergs cm ?2 s?1 (0.5-2.0 keV). Here we provide the cluster catalog and finder charts consisting of X-ray overlays on optical CCD images. We also compare our redshifts, fluxes, and detection methods to other similar published cluster surveys and find no serious issues with our measurements or completeness.

The evolution of the cluster X-ray scaling relations in the Wide Angle ROSAT Pointed Survey sample at 0.6 &lt; z &lt; 1.0

The X-ray properties of a sample of 11 high-redshift (0.6 < z < 1 .O) clusters observed with Chardm and/or XMM-Newton are used to investigate the evolution of the cluster scaling relations. The observed evolution in the normalization of the L-T, M-T, M(sub 2)-T and M-L relations is consistent with simple self-similar predictions, in which the properties of clusters reflect the properties of the Universe at their redshift of observation. Under the assumption that the model of self-similar evolution is correct and that the local systems formed via a single spherical collapse, the high-redshift L-T relation is consistent with the high-z clusters having virialized at a significantly higher redshift than the local systems. The data are also consistent with the more realistic scenario of clusters forming via the continuous accretion of material. The slope of the L-T relation at high redshift (B = 3.32 +/- 0.37) is consistent with the local relation, and significantly steeper than the self-similar prediction of B = 2. This suggests that the same non-gravitational processes are responsible for steepening the local and high-z relations, possibly occurring universally at z is approximately greater than 1 or in the early stages of the cluster formation, prior to their observation. The properties of the intracluster medium at high redshift are found to be similar to those in the local Universe. The mean surface-brightness profile slope for the sample is Beta = 0.66 +/- 0.05, the mean gas mass fractions within R(sub 2500(z)) and R(200(z)) are 0.069 +/- 0.012 and 0.11 +/- 0.02, respectively, and the mean metallicity of the sample is 0.28 +/- 0.11 Z(sub solar).

The WARPS Survey. VI. Galaxy Cluster and Source Identifications from Phase I

We present in catalog form the optical identifications for objects from the first phase of the Wide Angle ROSAT Pointed Survey (WARPS). WARPS is a serendipitous survey of relatively deep, pointed ROSAT observations for clusters of galaxies. The X-ray source detection algorithm used by WARPS is Voronoi Tessellation and Percolation (VTP), a technique which is equally sensitive to point sources and extended sources of low surface brightness. WARPS-I is based on the central regions of 86 ROSAT PSPC fields, covering an area of 16.2 square degrees. We describe here the X-ray source screening and optical identification process for WARPS-I, which yielded 34 clusters at 0.06 < z < 0.75. Twenty-two of these clusters form a complete, statistically well-defined sample drawn from 75 of these 86 fields, covering an area of 14.1 square degrees, with a flux limit of F(0.5 × 2.0 keV) = 6.5 × 10-14 erg cm-2 s-1. This sample can be used to study the properties and evolution of the gas, galaxy and dark matter content of clusters and to constrain cosmological parameters. We compare in detail the identification process and findings of WARPS to those from other recently published X-ray surveys for clusters, including RDCS, SHARC-Bright, SHARC-south, and the CfA 160 deg2 survey.

Discovery of a Very X-Ray Luminous Galaxy Cluster at [CLC][ITAL]z[/ITAL][/CLC] = 0.89 in the Wide Angle [ITAL]ROSAT[/ITAL] Pointed Survey

We report the discovery of the galaxy cluster Cl J1226.9+3332 in the Wide Angle ROSAT Pointed Survey (WARPS). At z = 0.888 and LX = 1.1 × 1045 h ergs s-1 (0.5-2.0 keV), Cl J1226.9+3332 is the most distant X-ray luminous cluster currently known. The mere existence of this system represents a huge problem for Ω0 = 1 world models.

The WARPS Survey. III. The Discovery of an X‐Ray Luminous Galaxy Cluster atz = 0.833 and the Impact of X‐Ray Substructure on Cluster Abundance Measurements

The Wide Angle ROSAT Pointed Survey team reviews the properties and history of the discovery of Cl J0152.7-1357, an X-ray luminous, rich cluster of galaxies at a redshift of z = 0.833. At LX = 8 × 1044 h ergs s-1 (0.5-2.0 keV) Cl J0152.7-1357 is the most X-ray luminous cluster known at redshifts z > 0.55. The high X-ray luminosity of the system suggests that massive clusters may begin to form at redshifts considerably greater than unity. This scenario is supported by the high degree of optical and X-ray substructure in Cl J0152.7-1357, which is similarly complex as that of other X-ray-selected clusters at comparable redshift and consistent with the hypothesized picture of cluster formation by mass infall along large-scale filaments.

The WARPS survey - IV. The X-ray luminosity-temperature relation of high-redshift galaxy clusters

We present a measurement of the cluster X-ray luminosity-temperature relation out to high redshift (z~0.8). Combined ROSAT PSPC spectra of 91 galaxy clusters detected in the Wide Angle ROSAT Pointed Survey (WARPS) are simultaneously fit in redshift and luminosity bins. The resulting temperature and luminosity measurements of these bins, which occupy a region of the high redshift L-T relation not previously sampled, are compared to existing measurements at low redshift in order to constrain the evolution of the L-T relation. We find a best fit to low redshift (z 1 keV, to be L proportional to T^(3.15\pm0.06). Our data are consistent with no evolution in the normalisation of the L-T relation up to z~0.8. Combining our results with ASCA measurements taken from the literature, we find eta=0.19\pm0.38 (for Omega_0=1, with 1 sigma errors) where L_Bol is proportional to (1 + z)^eta T^3.15, or eta=0.60\pm0.38 for Omega_0=0.3. This lack of evolution is considered in terms of the entropy-driven evolution of clusters. Further implications for cosmological constraints are also discussed.

The Warps X-Ray Survey of Galaxies, Groups, and Clusters

We have embarked on a survey of ROSAT PSPC archival data searching for all detected surface brightness enhancements due to sources in the innermost R ≤ 15′ of the PSPC field of view in the energy band 0.5–2.0 keV. This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. Accurate measurements of the high redshift XLF would allow the form of the XLF evolution to be determined via the position of the Schechter function break. This would help discriminate between luminosity and density evolution, and discriminate between different hierarchical models, e.g., those including a different mix of fundamental particles, a flat power spectrum of the initial fluctuations, and reheating of the intracluster gas at high redshifts.

The Warps Blazar Survey Searching for the Faintest X-Ray Selected Blazars

The WARPS (Wide-Angle ROSAT Pointed Survey) blazar survey is a deep X-ray search for BL Lac objects and flat-radio-spectrum quasars (FRSQs), drawn from a cross-correlation of serendipitous sources in the ROSAT PSPC database WGACAT (White et al. 1994) with the Green Bank 6 cm and 20 cm (Condon et al. 1989, Condon &amp; Broderick 1985), the Parkes radio (Bolton et al. 1979), and the Parkes-MIT-NRAO (Griffith &amp; Wright 1993, Wright et al. 1994, Griffith et al. 1994, 1995) catalogs. Our sample contains 165 new blazar candidates and 95 previously known blazars.

The Wide‐AngleROSATPointed X‐Ray Survey of Galaxies, Groups, and Clusters. I. Method and First Results

We have embarked on a survey of ROSAT PSPC archival data with the aim of detecting all significant surface brightness enhancements resulting from sources in the innermost R ≤ 15' of the PSPC field of view in the energy band 0.5-2.0 keV. This project is part of the Wide-Angle ROSAT Pointed Survey (WARPS), and it is designed primarily to measure the low-luminosity, high-redshift, X-ray luminosity function of galaxy clusters and groups. The approach we have chosen for source detection (Voronoi Tessellation and Percolation [VTP]) represents a significant advance over conventional methods, and it is particularly suited for the detection and accurate quantification of extended and/or low surface brightness emission that could otherwise be missed or wrongly interpreted. We also use energy-dependent exposure maps to estimate the fluxes of sources that can amount to corrections of as much as 15%. In an extensive optical follow-up program, we are identifying galaxies, groups, and clusters at redshifts ranging from z ~ 0.1 to z ~ 0.7. In this paper, we present our method and its calibration using simulated and real data. We present first results for an initial 91 fields (17.2 deg2) at detected fluxes greater than 3.5 × 10-14 ergs s-1 cm-2 (the WARPS-I survey). We find the sky density of extended objects to be in the range 2.8-4.0 ( ± 0.4) deg-2. A comparison with a point-source detection algorithm demonstrates that our VTP approach typically finds 1-2 more objects deg-2 to this detected flux limit, suggesting that the conventional method fails to detect a significant fraction of extended objects. The surface brightness limit of the WARPS cluster survey is ~1 × 10-15 ergs s-1 cm-2 arcmin-2, approximately 6 times lower than the Extended Medium Sensitivity Survey (EMSS). The WARPS Log N-Log S (which currently represents a lower limit) shows a significant excess over previous measurements for S 8 × 10-14 ergs s-1 cm-2. We attribute this mainly to a larger measured flux from extended sources as well as new detections of low surface brightness systems in the WARPS.