arXiv subjects

H. Ebeling

Publications and source records attributed to H. Ebeling.

At least 91 records · Page 5Linked to original sources

arXiv2003

Clusters in the Zone of Avoidance

The Clusters in the Zone of Avoidance (CIZA) project is producing the first statistically complete catalogue of X-ray selected galaxy clusters behind the Galactic plane. Current optically selected cluster catalogues have excluded a wide band of sky centered on the Milky Way (|b| < 20 deg) due to the high extinction and stellar obscuration at low Galactic latitudes. An X-ray search for cluster candidates overcomes the problems faced by optically selected catalogues, hence providing the first opportunity to explore the sky in the traditional Zone of Avoidance; principally, the Great Attractor region. When added to other X-ray selected cluster surveys such as the BCS and REFLEX, CIZA will complete the first unbiased, all-sky cluster catalogue. We here report on the progress of the CIZA survey as well as preliminary results of a dipole analysis which utilizes the current CIZA sample.

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arXiv2003

An XMM and Chandra view of massive clusters of galaxies to z=1

The X-ray properties of a sample of high redshift (z>0.6), massive clusters observed with XMM-Newton and Chandra are described, including two exceptional systems. One, at z=0.89, has an X-ray temperature of T=11.5 (+1.1, -0.9) keV (the highest temperature of any cluster known at z>0.6), an estimated mass of (1.4+/-0.2)x10^15 solar masses and appears relaxed. The other, at z=0.83, has at least three sub-clumps, probably in the process of merging, and may also show signs of faint filamentary structure at large radii,observed in X-rays. In general there is a mix of X-ray morphologies, from those clusters which appear relaxed and containing little substructure to some highly non-virialized and probably merging systems. The X-ray gas metallicities and gas mass fractions of the relaxed systems are similar to those of low redshift clusters of the same temperature, suggesting that the gas was in place, and containing its metals, by z=0.8. The evolution of the mass-temperature relation may be consistent with no evolution or with the ``late formation'' assumption. The effect of point source contamination in the ROSAT survey from which these clusters were selected is estimated, and the implications for the ROSAT X-ray luminosity function discussed.

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arXiv2003

The nature and space density of fossil groups of galaxies

We describe the properties of a sample of galaxy groups with very unusual distributions of galaxy luminosities. The most extreme example has an X-ray luminosity similar to that of the Virgo cluster but has a very low richness, with only one galaxy brighter than L*, compared with six in Virgo. That one galaxy, however, is optically more luminous than any galaxy in Virgo and has an optical luminosity as bright as many of the central cD galaxies in rich Abell clusters. The characteristic feature of the fossil groups we study is that most of the light arises from one dominant, central galaxy. We define a fossil system and, based on this definition, construct a small X-ray selected, flux-limited sample of fossil groups with well known selection criteria. We confirm that these systems are indeed groups of galaxies, but dominated by one central luminous giant elliptical galaxy and with few, or no, L* galaxies. We find that fossil systems represent 8%-20% of all systems of the same X-ray luminosity. Fossil groups are at least as numerous as all poor and rich clusters combined, and are thus a possible site for the formation of luminous central cluster galaxies before infall into clusters occurs. The fossil systems in our sample have significantly higher X-ray luminosities than normal groups of similar total optical luminosities (or similar X-ray temperature, where the latter can be measured). These enhanced X-ray luminosities may be due to relatively cool gas in the innermost regions or due to a low central gas entropy. We interpret fossil groups as old, undisturbed systems which have avoided infall into clusters, but where galaxy merging of most of the L* galaxies has occurred. An early formation epoch, before that of most groups, could explain low central gas entropies and high X-ray luminosities.

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arXiv2003

Discovery of an overdensity of faint red galaxies in the vicinity of the z=1.786 radio galaxy 3C 294

We report the discovery of an overdensity of faint red galaxies in the vicinity of the z=1.786 radio galaxy 3C 294. The overdensity, discovered in a 84 min Ks-band ISAAC/VLT image is significant at the 2.4 sigma level (compared to the local field density), and overlaps with the extended X-ray emission around 3C 294 detected with the Chandra X-ray Observatory. The near-infrared colours of the galaxies making up the overdensity show a large scatter and the galaxies do not follow a red sequence in the colour magnitude diagram. If the galaxies are in a cluster at z=1.786 they must be dominated by young stellar populations with different star-formation histories.

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arXiv2003

Chandra X-ray analysis of the massive high-redshift galaxy clusters ClJ1113.1-2615 and ClJ0152.7-1357

We present an analysis of Chandra observations of two high-redshift clusters of galaxies, ClJ1113.1-2615 at z=0.725 and ClJ0152.7-1357 at z=0.833. We find ClJ1113 to be relaxed with kT=4.3^{+0.5}_{-0.4}keV and a mass (within the virial radius) of 4.3^{+0.8}_{-0.7}*10^{14}Msol. ClJ0152, by contrast, is resolved into a northern and southern subcluster, each massive and X-ray luminous, in the process of merging. The temperatures of the subclusters are found to be 5.5^{+0.9}_{-0.8}keV and 5.2^{+1.1}_{-0.9}keV respectively, and their respective masses are 6.1^{+1.7}_{-1.5}*10^{14}Msol and 5.2^{+1.8}_{-1.4}*10^{14}Msol within the virial radii. 2D modelling of the X-ray surface brightness reveals excess emission between the subclusters; suggestive, but not conclusive evidence of a shock front. We make a first attempt at measuring the cluster M-T relation at z~0.8, and find no evolution in its normalisation, supporting the previous assumption of an unevolving M-T relation. We also find little or no evolution in the L-T relation, the gas fraction-T relation, the beta-T relation or the metallicity. These results suggest that, in at least some massive clusters, the hot gas was in place, and containing its metals, at z~0.8. We also highlight the need to correct for the degradation of the Chandra ACIS low energy quantum efficiency in high-redshift cluster studies when the low energy absorption is often assumed to be the Galactic value, rather than measured.

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arXiv2003

Cosmological constraints from the local X-ray luminosity function of the most X-ray luminous galaxy clusters

We present precise constraints on the normalization of the power spectrum of mass fluctuations in the nearby universe, sigma_8, as a function of the mean local matter density, Omega_m. Using the observed local X-ray luminosity function of galaxy clusters from the extended BCS and REFLEX studies, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by the Hubble Volume simulations of Evrard et al., we obtain sigma_8 = (0.508\pm0.019) Omega_m^-(0.253\pm0.024), with Omega_m < 0.34 at 68 per cent confidence. The degeneracy between sigma_8 and Omega_m can be broken using Chandra measurements of the X-ray gas mass fractions in dynamically relaxed clusters. Using this information and including Gaussian priors on the mean baryon density of the universe and the Hubble constant, we obtain sigma_8=0.695\pm0.042 and Omega_m=0.287\pm0.036, for an assumed flat LambdaCDM cosmology (marginalized 68 per cent confidence limits). Our results are in good agreement with some recent studies based on the local X-ray temperature function of clusters, the redshift evolution of the X-ray luminosity and temperature functions of clusters, early results from the Sloan Digitized Sky Survey, the most recent results from studies of cosmic shear, and combined analyses of the 2dF galaxy redshift survey and cosmic microwave background anisotropies.

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arXiv2002

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 $F (0.5-2.0 keV) = 6.5 \times 10^{-14} erg cm^{-2} s^{-1}}$. 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 deg$^2$ survey.

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arXiv2002

Time-Dependent Optical Spectroscopy of GRB 010222: Clues to the GRB Environment

We present sequential optical spectra of the afterglow of GRB 010222 obtained one day apart using the Low Resolution Imaging Spectrometer (LRIS) and the Echellette Spectrograph and Imager (ESI) on the Keck telescopes. Three low-ionization absorption systems are spectroscopically identified at z1=1.47688, z2=1.15628, and z3=0.92747. The higher resolution ESI spectrum reveals two distinct components in the highest redshift system at z1a=1.47590 and z1b=1.47688. We interpret the z1b=1.47688 system as an absorption feature of the disk of the host galaxy of GRB 010222. The best fitted power-law optical continuum and [Zn/Cr] ratio imply low dust content or a local gray dust component near the burst site. In addition, we do not detect strong signatures of vibrationally excited states of H_2. If the GRB took place in a superbubble or young stellar cluster, there are no outstanding signatures of an ionized absorber, either. Analysis of the spectral time dependence at low resolution shows no significant evidence for absorption-line variability. This lack of variability is confronted with time-dependent photoionization simulations designed to apply the observed flux from GRB 010222 to a variety of assumed atomic gas densities and cloud radii. The absence of time dependence in the absorption lines implies that high-density environments are disfavored. In particular, if the GRB environment was dust free, its density was unlikely to exceed nH=10^2 cm^-3. If depletion of metals onto dust is similar to Galactic values or less than solar abundances are present, then nH > 2 x 10^4 cm^-3 is probably ruled out in the immediate vicinity of the burst.

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arXiv2002

A Hubble Space Telescope Lensing Survey of X-ray Luminous Galaxy Clusters: III. A Multiply-imaged Extremely Red Galaxy at z=1.6

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of EROJ003707+0909.5, the brightest of three gravitationally-lensed images of an Extremely Red Object (ERO) at z=1.6, in the field of the massive cluster A68 (z=0.255). We exploit the superlative resolution of our HST data and the enhanced spatial resolution and sensitivity afforded by the lens amplification to reconstruct the source-plane properties of this ERO. Our morphological and photometric analysis reveals that EROJ003707 is an L* early-type disk-galaxy and we estimate that ~10 per cent of EROs with (R-K)>=5.3 and K<=21 may have similar properties. The unique association of passive EROs with elliptical galaxies therefore appears to be too simplistic. We speculate on the evolution of EROJ003707: if gas continues to cool onto this galaxy in the manner predicted by hierarchical galaxy formation models, then by the present day, EROJ003707 could evolve into a very luminous spiral galaxy.

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arXiv2002

The ROSAT All-Sky Survey: a Catalog of Clusters of Galaxies in a Region of 1 Ster around the South Galactic Pole

A field of 1.013 ster in the ROSAT all-sky survey (RASS), centered on the south galactic pole (SGP), has been searched in a systematic, objective manner for clusters of galaxies. The procedure relied on a correlation of the X-ray positions and properties of ROSAT sources with the distribution of galaxies in the COSMOS digitised data base, which was obtained by scanning the plates of the UK Schmidt IIIa-J optical southern sky survey. The study used the second ROSAT survey data base (RASS-2) and included several optical observing campaigns to measure redshifts. The search, a precursor to the larger REFLEX survey of the whole southern sky, reached the detection limits of the RASS and the COSMOS data, and yielded a catalog of 186 clusters in which the lowest flux is 1.5e-12 erg/cm2/s in the 0.1-2.4 keV band. Of these 157 have measured redshifts. Using a limit of 3.0e-12 erg/cm2/s a complete subset of 112 clusters was obtained, of which 110 have measured redshifts. The spatial distribution of the X-ray clusters out to z = 0.15 shows an extension of the Local Supercluster to the Pisces-Cetus supercluster (z<~0.07), and a more distant orthogonal structure at 0.07<z<0.15.

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arXiv2001

Distinguishing Local and Global Influences on Galaxy Morphology: An HST Comparison of High and Low X-ray Luminosity Clusters

(Abridged) We present a morphological analysis of 17 X-ray selected clusters at z~0.25, imaged uniformly with HST WFPC2. Eight of these clusters have low X-ray luminosities (<10^{44} erg/s), while the remaining nine clusters have L_x>10^{45} ergs/s. The clusters cover a relatively small range in redshift and the data are homogeneous in terms of depth, resolution and rest wavelength observed. We use GIM2D to fit the two dimensional surface brightness profiles of galaxies down to M(702)<=-18.2 (Ho=50; roughly 0.01 L*) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image we find that the Low-Lx clusters are dominated by galaxies with low B/T (~0), while the High-Lx clusters are dominated by galaxies with intermediate B/T (~0.4). We test whether this difference could arise from a universal morphology-density relation due to differences in the typical galaxy densities in the two samples. We find that small differences in the B/T distributions of the two samples persist with marginal statistical significance even when we restrict the comparison to galaxies in environments with similar projected local galaxy densities. From the correlations with the bulge and disk luminosity functions, we argue that the global environment affects the population of bulges, over and above trends seen with local density. We conclude that the destruction of disks through ram pressure stripping or harassment is not solely responsible for the morphology-density relation, and that bulge formation is less efficient in low mass clusters.

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arXiv2001

MACS: A quest for the most massive galaxy clusters in the universe

We describe the design and current status of a new X-ray cluster survey aimed at the compilation of a statistically complete sample of very X-ray luminous (and thus, by inference, massive), distant clusters of galaxies. The primary goal of the MAssive Cluster Survey (MACS) is to increase the number of known massive clusters at z>0.3 from a handful to hundreds. Upon completion of the survey, the MACS cluster sample will greatly improve our ability to study quantitatively the physical and cosmological parameters driving cluster evolution at redshifts and luminosities poorly sampled by all existing surveys. To achieve these goals we apply an X-ray flux and X-ray hardness-ratio cut to select distant cluster candidates from the ROSAT Bright Source Catalogue. Starting from a list of more than 5,000 X-ray sources within the survey area of 22,735 square degrees we use positional cross-correlations with public catalogues of Galactic and extragalactic objects, reference to APM colours, visual inspection of Digitized Sky Survey images, extensive CCD imaging, and finally spectroscopic observations with the University of Hawaii's 2.2m and the Keck 10m telescopes to compile the final cluster sample. We discuss in detail the X-ray selection procedure and the resulting selection function, and present model predictions for the number of distant clusters expected to emerge from MACS. At the time of this writing the MACS cluster sample comprises 101 spectroscopically confirmed clusters at 0.3<z<0.6; more than two thirds of these are new discoveries. Our preliminary sample is already 15 times larger than that of the EMSS in the same redshift and X-ray luminosity range.

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arXiv2000

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 ClJ1226.9+3332 in the Wide Angle ROSAT Pointed Survey (WARPS). At z=0.888 and L_X=1.1e45 erg/s (0.5-2.0 keV, h_0=0.5) ClJ1226.9+3332 is the most distant X-ray luminous cluster currently known. The mere existence of this system represents a huge problem for Omega_0=1 world models. At the modest (off-axis) resolution of the ROSAT PSPC observation in which the system was detected, ClJ1226.9+3332 appears relaxed; an off-axis HRI 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, 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-Zel'dovich effect in its direction (Joy et al. 2001). Using ClJ1226.9+3332 and ClJ0152.7-1357 (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 5e44 erg/s. 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<0.3), and conclude that negative evolution is not required at these luminosities out to z~1. (truncated)

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arXiv2000

The X-ray evolution of clusters of galaxies to z=0.9

The evolution of the X-ray luminosity function of clusters of galaxies has been measured to z=0.9 using over 150 X-ray selected clusters discovered in the WARPS survey. We find no evidence for evolution of the luminosity function at any luminosity or redshift. The observations constrain the evolution of the space density of moderate luminosity clusters to be very small, and much less than predicted by most models of the growth of structure with Omega_m=1. All the current X-ray surveys agree on this result. Several notable luminous clusters at z>0.8 have been found, including one cluster which is more luminous (and is probably more massive) than the well known MS1054 cluster.

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arXiv2000

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<0.2) cluster data, at T>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.

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arXiv2000

The ROSAT Brightest Cluster Sample (BCS) - IV. The extended sample

We present a low-flux extension of the X-ray selected ROSAT Brightest Cluster Sample (BCS) published in Paper I of this series. Like the original BCS and employing an identical selection procedure, the BCS extension is compiled from ROSAT All-Sky Survey (RASS) data in the northern hemisphere (dec > 0 deg) and at high Galactic latitudes (|b| > 20 deg). It comprises 100 X-ray selected clusters of galaxies with measured redshifts z < 0.3 (as well as seven more at z > 0.3) and total fluxes between 2.8 x 10-12 erg cm-2 s-1 and 4.4 x 10-12 erg cm-2 s-1 in the 0.1-2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the main sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to emerge from the ROSAT All-Sky Survey to date. The nominal completeness of the combined sample (defined with respect to a power law fit to the bright end of the BCS log N-log S distribution) is relatively low at 75 per cent (compared to 90 per cent for the high-flux sample of Paper I). However, just as for the original BCS, this incompleteness can be accurately quantified, and thus statistically corrected for, as a function of X-ray luminosity and redshift. In addition to its importance for improved statistical studies of the properties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray bright clusters in the northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra or XMM-Newton.

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arXiv2000

Evolution of the cluster X-ray luminosity function from the WARPS survey

The evolution of the X-ray luminosity function of clusters of galaxies has been measured to z=0.85 using over 150 X-ray selected clusters found in the WARPS survey. We find no evidence for evolution of the luminosity function at any luminosity or redshift. The observations constrain the evolution of the space density of moderate luminosity clusters to be very small, and much less than predicted by most models of the growth of structure with Omega=1.

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arXiv1999

The WARPS survey: III. The discovery of an X-ray luminous galaxy cluster at z=0.833 and the impact of X-ray substructure on cluster abundance measurements

The WARPS team reviews the properties and history of discovery of ClJ0152.7-1357, an X-ray luminous, rich cluster of galaxies at z=0.833. At L_X = 8 x 10^44 h^(-2) erg/s (0.5-2.0 keV) ClJ0152.7-1357 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 ClJ0152.7-1357, which is similarly complex as that of other X-ray selected distant clusters and consistent with the picture of cluster formation by mass infall along large-scale filaments. X-ray emission from ClJ0152.7-1357 was detected already in 1980 with the EINSTEIN IPC. However, because the complex morphology of the emission caused its significance to be underestimated, the corresponding source was not included in the EMSS cluster sample and hence not previously identified. Simulations of the EMSS source detection and selection procedure suggest a general bias of the EMSS against X-ray luminous clusters with pronounced substructure. If highly unrelaxed, merging clusters are common at high redshift, they could create a bias in some samples as the morphological complexity of mergers may cause them to fall below the flux limit of surveys that assume a unimodal spatial source geometry. Conversely, the enhanced X-ray luminosity of mergers might cause them to, temporarily, rise above the flux limit. Either effect could lead to erroneous conclusions about the evolution of the comoving cluster space density. A high fraction of morphologically complex clusters at high redshift would also call into question the validity of cosmological studies that assume that the systems under investigation are virialized.

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