Donald John Horner

The WARPS Survey. II. The log N-log S Relation and the X-Ray Evolution of Low-Luminosity Clusters of Galaxies

The strong negative evolution observed in previous X-ray-selected surveys of clusters of galaxies is evidence in favor of hierarchical models of the growth of structure in the universe. A large recent survey has, however, contradicted the low-redshift results, finding no evidence for evolution at z 0.3) the cluster luminosities are in the range 4 × 1043-2 × 1044 h−250 ergs s-1, the luminosities of poor clusters. The number of high-redshift, low-luminosity clusters is consistent with no evolution of the X-ray luminosity function between redshifts of z ≈ 0.4 and z = 0, and it places a limit of a factor of less than 1.7 (at 90% confidence) on the amplitude of any pure negative density evolution of clusters of these luminosities, in contrast with the factor of ≈ 3 [corresponding to number density evolution ∝(1 + z)-2.5] found in the Einstein Extended Medium-Sensitivity Survey at similar redshifts but higher luminosities. Taken together, these results support hierarchical models in which there is mild negative evolution of the most luminous clusters at high redshift, but little or no evolution of the less luminous but more common optically poor clusters. Models involving preheating of the X-ray gas at an early epoch fit the observations, at least for Ω0 = 1.

Entropy Profiles in the Cores of Cooling Flow Clusters of Galaxies

The X-ray properties of a relaxed cluster of galaxies are determined primarily by its gravitational potential well and the entropy distribution of its intracluster gas. That entropy distribution reflects both the accretion history of the cluster and the feedback processes that limit the condensation of intracluster gas. Here we present Chandra observations of the core entropy profiles of nine classic cooling flow clusters that appear relatively relaxed (at least outside the central 10-20 kpc) and contain intracluster gas with a cooling time less than a Hubble time. We show that those entropy profiles are remarkably similar, despite the fact that the clusters range over a factor of 3 in temperature. They typically have an entropy level of ?130 keV cm2 at 100 kpc that declines to a plateau ~10 keV cm2 at 10 kpc. Between these radii, the entropy profiles are r? with ? ? 1.0-1.3. The nonzero central entropy levels in these clusters correspond to a cooling time ~108 yr, suggesting that episodic heating on this timescale maintains the central entropy profile in a quasi-steady state. We show in an appendix that although disturbances and bubbles are visible in the central regions of these clusters, these phenomena do not strongly bias our entropy estimates.

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.

Constraints on the Cluster Environments and Hot Spot Magnetic Field Strengths of the Radio Sources 3C 280 and 3C 254

We present new Chandra Observatory observations together with archival Hubble Space Telescope (HST) and radio observations of 3C 254, a radio quasar at z = 0.734, and 3C 280, a radio galaxy at z = 0.996. We report the detection of X-ray and possible HST optical counterparts to the radio hot spots in 3C 280 and the detection of an X-ray counterpart to the western radio hot spot in 3C 254. We present constraints on the presence of X-ray clusters and on the magnetic field strengths in and around the radio hot spots for both targets. Both sources were thought to be in clusters of galaxies based on reports of significant extended emission in ROSAT PSPC and HRI images. The exquisite spatial resolution of the Chandra Observatory allows us to demonstrate that these sources are not in hot, massive clusters. The extended emission seen in ROSAT observations is resolved by Chandra into point sources and is likely to be X-ray emission associated with the radio hot spots of these sources, with possible additional contributions from unrelated point sources. The intergalactic medium around these sources could be dense, but it is demonstrably not dense and hot. We conclude that radio sources are not reliable signposts of massive clusters at moderately high redshifts. We present measurements of the X-ray and optical fluxes of source features and discuss what physical processes may give rise to them. X-ray synchrotron emission could explain the radio, optical, and X-ray hot spot fluxes in 3C 280; this would require continuous acceleration of electrons to high Lorentz factors since the synchrotron lifetime of relativistic electrons that could produce the X-ray emission would be of the order of a human lifetime. Synchrotron self-Compton emission with or without inverse Compton emission due to scattering of the cosmic microwave background radiation can also explain the X-ray emission from the hot spots, although this most likely would require that some other physical process be invoked to explain the optical emission seen in 3C 280. High spatial resolution radio data with broad frequency coverage of the radio hot spot regions are needed to determine which physical process is responsible for the detected X-ray emission and to provide much tighter constraints on the magnetic field strength of the hot spot plasma. We summarize our current constraints on the magnetic field strengths in and around the hot spots of 3C 254 and 3C 280.

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.

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. At the modest (off-axis) resolution of the ROSAT Position Sensitive Proportional Counter observation in which the system was detected, Cl J1226.9+3332 appears relaxed; an off-axis High Resolution Imager observation confirms this impression and rules out significant contamination from point sources. However, in moderately deep optical images (R and I band), the cluster exhibits signs of substructure in its apparent galaxy distribution. A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km s-1, indicative of a very massive cluster and/or the presence of substructure along the line of sight. While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths, the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zeldovich effect in its direction. Using Cl J1226.9+3332 and Cl J0152.7-1357 (z = 0.835), the second most distant X-ray luminous cluster currently known and also a WARPS discovery, we obtain a first estimate of the cluster X-ray luminosity function at 0.8 5 × 1044 h ergs s-1 (0.5-2.0 keV). Using the best currently available data, we find the comoving space density of very distant, massive clusters to be in excellent agreement with the value measured locally (z 3 σ) negative evolution already at 0.3 < z < 0.6 based on the cluster samples of the Einstein Extended Medium-Sensitivity Survey (EMSS) and the Center for Astrophysics 160° survey. Our results agree, however, with the lack of significant evolution of very X-ray luminous clusters out to z ~ 0.4 reported by the MAssive Cluster Survey team. Our findings are also consistent with the abundance of very X-ray luminous clusters at z ~ 0.8 inferred from the EMSS cluster sample, provided Cl J0152.7-1357 (which was missed by the EMSS) is added in.

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 nal sample of 124 clusters, complete above a ux limit of 6 :5 10 14 erg cm 2 s 1 , with members out to redshift z 1:05, and a sky coverage of 70.9 deg 2 . We nd signicant 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 aect 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 nd that the detected evolution remains signicant at the 95% level. We observe a signicant excess of clusters in the WARPS at 0:1 < z < 0:3 and LX 2 10 43 erg s 1 compared with the reference low-redshift XLF, or our Bayesian t to the WARPS data. We nd 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.

Discovery of a very x-ray luminous galaxy cluster at z=0.89 in the warps 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. At the modest (off-axis) resolution of the ROSAT Position Sensitive Proportional Counter observation in which the system was detected, Cl J1226.9+3332 appears relaxed; an off-axis High Resolution Imager observation confirms this impression and rules out significant contamination from point sources. However, in moderately deep optical images (R and I band), the cluster exhibits signs of substructure in its apparent galaxy distribution. A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km s-1, indicative of a very massive cluster and/or the presence of substructure along the line of sight. While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths, the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zeldovich effect in its direction. Using Cl J1226.9+3332 and Cl J0152.7-1357 (z = 0.835), the second most distant X-ray luminous cluster currently known and also a WARPS discovery, we obtain a first estimate of the cluster X-ray luminosity function at 0.8 5 × 1044 h ergs s-1 (0.5-2.0 keV). Using the best currently available data, we find the comoving space density of very distant, massive clusters to be in excellent agreement with the value measured locally (z 3 σ) negative evolution already at 0.3 < z < 0.6 based on the cluster samples of the Einstein Extended Medium-Sensitivity Survey (EMSS) and the Center for Astrophysics 160° survey. Our results agree, however, with the lack of significant evolution of very X-ray luminous clusters out to z ~ 0.4 reported by the MAssive Cluster Survey team. Our findings are also consistent with the abundance of very X-ray luminous clusters at z ~ 0.8 inferred from the EMSS cluster sample, provided Cl J0152.7-1357 (which was missed by the EMSS) is added in.

X‐Ray Analysis of the MS0302 Supercluster

The MS0302 supercluster is comprised of three massive clusters at z = 0.42 (GHO 0303+170, MS 0302.7+1658, and MS0302.5+1717). While this supercluster has been the subject of deep photometric and lensing studies, it has been rather poorly observed in the X‐rays. We are conducting an analysis of existing ROSAT HRI, ASCA, and Chandra data of supercluster. Here, we present a new mosaic of the region using existing ASCA observations of the region and X‐ray temperatures for GHO 0303+170 and MS0302.5+1717.

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 0.5–2.0keV. 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.