J. P. Henry

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 5000 X-ray sources within the survey area of 22,735 deg2, we use positional cross-correlations with public catalogs of Galactic and extragalactic objects, reference to Automated Plate Measuring Machine (APM) colors, visual inspection of Digitized Sky Survey images, extensive CCD imaging, and finally spectroscopic observations with the University of Hawaiis 2.2 m and the Keck 10 m 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.

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

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.

The 160 Square Degree ROSAT Survey: The revised catalog of 201 clusters with spectroscopic redshifts

We present the revised catalog of galaxy clusters detected as extended X-ray sources in the 160 Square Degree ROSAT Survey, including spectroscopic redshifts and X-ray luminosities for 200 of the 201 members. The median redshift is zmedian = 0.25, and the median X-ray luminosity is LX,median = 4.2 × 1043 h ergs s-1 (0.5-2.0 keV). This is the largest high-redshift sample of X-ray-selected clusters published to date. There are 73 objects at z > 0.3 and 22 objects at z > 0.5 drawn from a statistically complete flux-limited survey with a median object flux of 1.4 × 10-13 ergs cm-2 s-1. We describe the optical follow-up of these clusters with an emphasis on our spectroscopy, which has yielded 155 cluster redshifts, 110 of which are presented here for the first time. These measurements, combined with 45 from the literature and other sources, provide near-complete spectroscopic coverage for our survey. We discuss the final optical identifications for the extended X-ray sources in the survey region and compare our results to similar X-ray cluster searches.

XMM-NEWTON DISCOVERY OF AN X-RAY FILAMENT IN COMA

XMM-Newton observations of the outskirts of the Coma cluster of galaxies confirm the existence of warm X-ray gas claimed previously and provide a robust estimate of its temperature (~ 0.2 keV) and oxygen abundance (~ 0.1 solar). Associating this emission with a 20 Mpc infall region in front of Coma, seen in the skewness of its galaxy velocity distribution, yields an estimate of the density of the warm gas of ~ 50 Pcritical . Our measurements of gas mass associated with the warm emission strongly support its nonvirialized nature, suggesting that we are observing the warm-hot intergalactic medium (WHIM). Our measurements provide a direct estimate of the O, Ne and Fe abundance of the WHIM. Differences with the reported Ne/O ratio for some OVI absorbers hints to different origin of the OVI absorbers and the Coma filament.

Chandra X-Ray Detection of the Radio Hot Spots of 3C 295

An observation of the radio galaxy 3C 295 during the calibration phase of the Chandra X-Ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hot spots, and from the previously known cluster gas. We discuss the possible emission processes for the hot spots and argue that a synchrotron self-Compton (SSC) model is preferred for most or all of the observed X-ray emission. SSC models with near-equipartition fields thus explain the X-ray emission from the hot spots in the two highest surface brightness FR II radio galaxies, Cygnus A and 3C 295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

Weak Lensing by Two z ~ 0.8 Clusters of Galaxies

We report the detection of weak gravitational lensing of distant background galaxies by two rich, X-ray-luminous clusters of galaxies, MS 1137+66 at z=0.783 and RXJ 1716+67 at z = 0.813. We detect a shear signal in each cluster over the radial range 100≤r ≤ 700 h−1 kpc. Assuming that the background galaxies lie in a sheet at z = 2, we measure a mass of 2.45 × 1014 h−1 M☉ and M/LV = 270 at 500 h−1 kpc radius for MS 1137+66 and a mass of 2.6 × 1014 h−1 M☉ and M/LV = 190 for RXJ 1716+67 at the same radius. Both the optical light maps and the weak lensing mass maps of RXJ 1716+67 show two spatially distinct subclusters, as well as a long filamentary structure extending out of the cluster to the northeast. In contrast, MS 1137+66 is an ultracompact massive cluster in both mass maps and optical light maps and contains a strongly lensed arc system in the cluster core. These data add to the growing number of massive clusters at z 0.8.

Cosmological Constraints from the Evolution of the Cluster Baryon Mass Function

We determine the evolution of the number density of the galaxy clusters between z=0 and z=0.5. Our method uses the cluster baryon mass as a proxy for the total mass, thereby avoiding any uncertainties of the M tot−T or M tot−L X relations. Instead, we rely on a well-founded assumption that the M b /M tot ratio is a universal quantity. Taking advantage of direct and accurate Chandra measurements of the gas masses for distant clusters, we find strong evolution of the baryon mass function between z>0.4 and the present. The observed evolution defines a narrow band in the Ωm−Λ plane which intersects with constraints from the Cosmic Microwave Background and supernovae Ia near Ωm=0.3 and Λ=0.7. Assuming the flat Universe, cluster evolution favors the values of the dark energy equation of state parameter, w≈−1.

RX J1716.6+6708: A Young Cluster at z = 0.81

Clusters of galaxies at redshifts nearing 1 are of special importance since they may be caught at the epoch of formation. At these high redshifts there are very few known clusters. We present follow-up ASCA, ROSAT High Resolution Imager, and Keck LRIS observations of the cluster RX J1716.6+6708, which was discovered during the optical identification of X-ray sources in the north ecliptic pole region of the ROSAT All-Sky Survey. At z = 0.809, RX J1716.6+6708 is the second most distant X-ray–selected cluster so far published and the only one with a large number of spectroscopically determined cluster member velocities. The optical morphology of RX J1716.6+6708 resembles an inverted S-shape filament with the X-rays coming from the midpoint of the filament. The X-ray contours have an elongated shape that roughly coincides with the weak lensing contours. The cluster has a low temperature, kT = 5.66 keV, and a very high velocity dispersion σlos = 1522 km s-1. While the temperature is commensurate with its X-ray luminosity of (8.19 ± 0.43) × 1044 h ergs s-1 (2–10 keV rest frame), its velocity dispersion is much higher than expected from the σ-TX relationship of present-day clusters with comparable X-ray luminosity. RX J1716.6+6708 could be an example of a protocluster, where matter is flowing along filaments and the X-ray flux is maximum at the impact point of the colliding streams of matter.

Spatial correlation function of X-ray-selected active galactic nuclei

We present a detailed description of the first direct measurement of the spatial correlation function of X-ray-selected active galactic nuclei (AGNs). This result is based on an X-ray flux-limited sample of 219 AGNs discovered in the contiguous 80.7 deg2 region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at the 4 σ level at comoving scales in the interval r = 5-60 h-1 Mpc. Fitting the data with a power law of slope γ = 1.8, we find a correlation length of r0 = 7.4 h-1 Mpc (ΩM = 0.3, ΩΛ = 0.7). The median redshift of the AGN contributing to the signal is zξ = 0.22. This clustering amplitude implies that X-ray-selected AGNs are spatially distributed in a manner similar to that of optically selected AGNs. Furthermore, the ROSAT NEP determination establishes the local behavior of AGN clustering, a regime that is poorly sampled in general. Combined with high-redshift measures from optical studies, the ROSAT NEP results argue that the AGN correlation strength essentially does not evolve with redshift, at least out to z ~ 2.2. In the local universe, X-ray-selected AGNs appear to be unbiased relative to galaxies, and the inferred X-ray bias parameter is near unity, bX ~ 1. Hence, X-ray-selected AGNs closely trace the underlying mass distribution. The ROSAT NEP AGN catalog, presented here, features complete optical identifications and spectroscopic redshifts. The median redshift, X-ray flux, and X-ray luminosity are z = 0.41, fX = 1.1 × 10-13 ergs cm-2 s-1, and LX = 9.2 × 1043 h ergs s-1 (0.5-2.0 keV), respectively. Unobscured, type I AGNs are the dominant constituents (90%) of this soft X-ray-selected sample of AGNs.

EVOLUTION OF THE CLUSTER X-RAY LUMINOSITY FUNCTION

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.