L. Van Speybroeck

Erratum: "Chandra Sample of Nearby Relaxed Galaxy Clusters: Mass, Gas Fraction, and Mass-Temperature Relation"

We present gas and total mass profiles for 13 low-redshift, relaxed clusters spanning a temperature range 0.7-9 keV, derived from all available Chandra data of sufficient quality. In all clusters, gas-temperature profiles are measured to large radii (Vikhlinin et al.) so that direct hydrostatic mass estimates are possible to nearly r500 or beyond. The gas density was accurately traced to larger radii; its profile is not described well by a beta model, showing continuous steepening with radius. The derived ρtot profiles and their scaling with mass generally follow the Navarro-Frenk-White model with concentration expected for dark matter halos in ΛCDM cosmology. However, in three cool clusters, we detect a central mass component in excess of the Navarro-Frenk-White profile, apparently associated with their cD galaxies. In the inner region (r < 0.1r500), the gas density and temperature profiles exhibit significant scatter and trends with mass, but they become nearly self-similar at larger radii. Correspondingly, we find that the slope of the mass-temperature relation for these relaxed clusters is in good agreement with the simple self-similar behavior, M500 Tα, where α = (1.5-1.6) ± 0.1, if the gas temperatures are measured excluding the central cool cores. The normalization of this M-T relation is significantly, by ≈30%, higher than most previous X-ray determinations. We derive accurate gas mass fraction profiles, which show an increase with both radius and cluster mass. The enclosed fgas profiles within r2500 0.4r500 have not yet reached any asymptotic value and are still far (by a factor of 1.5-2) from the universal baryon fraction according to the cosmic microwave background (CMB) observations. The fgas trends become weaker and its values closer to universal at larger radii, in particular, in spherical shells r2500 < r < r500.

Constraints on dark energy from Chandra observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, {Omega}{sub m}, dark energy density, {Omega}{sub DE}, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas) in 42 hot (kT > 5keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1.1. Using only the fgas data for the 6 lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measure {Omega}{sub m}=0.28{+-}0.06 (68% confidence, using standard priors on the Hubble Constant, H{sub 0}, and mean baryon density, {Omega}{sub b}h{sup 2}). Analyzing the data for all 42 clusters, employing only weak priors on H{sub 0} and {Omega}{sub b}h{sup 2}, we obtain a similar result on {Omega}{sub m} and detect the effects of dark energy on the distances to the clusters at {approx}99.99% confidence, with {Omega}{sub DE}=0.86{+-}0.21 for a non-flat LCDM model. The detection of dark energy is comparable in significance to recent SNIa studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the f{sub gas} data, despite a weighted mean statistical scatter in the distance measurements of only {approx}5%. For a flat cosmology with constant w, we measure {Omega}{sub m}=0.28{+-}0.06 and w=-1.14{+-}0.31. Combining the fgas data with independent constraints from CMB and SNIa studies removes the need for priors on {Omega}{sub b}h{sup 2} and H{sub 0} and leads to tighter constraints: {Omega}{sub m}=0.253{+-}0.021 and w=-0.98{+-}0.07 for the same constant-w model. More general analyses in which we relax the assumption of flatness and/or allow evolution in w remain consistent with the cosmological constant paradigm. Our analysis includes conservative allowances for systematic uncertainties. The small systematic scatter and tight constraints bode well for future dark energy studies using the f{sub gas} method.

Chandra temperature profiles for a sample of nearby relaxed galaxy clusters

We present Chandra gas temperature profiles at large radii for a sample of 13 nearby, relaxed galaxy clusters and groups, which includes A133, A262, A383, A478, A907, A1413, A1795, A1991, A2029, A2390, MKW 4, RX J1159+5531, and USGC S152. The sample covers a range of average temperatures from 1 to 10 keV. The clusters are selected from the archive or observed by us to have sufficient exposures and off-center area coverage to enable accurate background subtraction and reach the temperature accuracy of better than 20%-30% at least to r = (0.4-0.5)r180 and for the three best clusters to (0.6-0.7)r180. For all clusters, we find cool gas in the cores, outside of which the temperature reaches a peak at r ~ 0.15r180 and then declines to ~0.5 of its peak value at r 0.5r180. When the profiles are scaled by the cluster average temperature (excluding cool cores) and the estimated virial radius, they show large scatter at small radii but remarkable similarity at r > (0.1-0.2)r180 for all but one cluster (A2390). Our results are in good agreement with previous measurements from ASCA by Markevitch et al. and from BeppoSAX by De Grandi & Molendi. Four clusters have recent XMM-Newton temperature profiles, two of which agree with our results, and we discuss reasons for disagreement for the other two. The overall shape of the temperature profiles at large radii is reproduced in recent cosmological simulations.

Chandra Observation of Abell 2142: Survival of Dense Subcluster Cores in a Merger

We use Chandra data to map the gas temperature in the central region of the merging cluster A2142. The cluster is markedly nonisothermal; it appears that the central cooling flow has been disturbed but not destroyed by a merger. The X-ray image exhibits two sharp, bow-shaped, shocklike surface brightness edges or gas density discontinuities. However, temperature and pressure profiles across these edges indicate that these are not shock fronts. The pressure is reasonably continuous across these edges, while the entropy jumps in the opposite sense to that in a shock (i.e., the denser side of the edge has lower temperature, and hence lower entropy). Most plausibly, these edges delineate the dense subcluster cores that have survived a merger and ram pressure stripping by the surrounding shock-heated gas.

Images, Structural Properties, and Metal Abundances of Galaxy Clusters Observed with Chandra ACIS-I at 0.1 < z < 1.3

We have assembled a sample of 115 galaxy clusters at 0.1 0.5. The slope of the surface brightness profiles at large radii were steeper on average by 15% than the slope obtained by fitting a simple β-model to the emission. This slope was also found to be correlated with cluster temperature, with some indication that the correlation is weaker for the clusters at z > 0.5. We measured the mean metal abundance of the cluster gas as a function of redshift and found significant evolution, with the abundances dropping by 50% between z = 0.1 and z ≈ 1. This evolution was still present (although less significant) when the cluster cores were excluded from the abundance measurements, indicating that the evolution is not solely due to the disappearance of relaxed, cool core clusters (which are known to have enhanced core metal abundances) from the population at z 0.5.

A complete sample of 12 very x-ray luminous galaxy clusters at z >0.5

We present the statistically complete and cosmologically most relevant subset of the 12 most distant galaxy clusters detected at z > 0.5 by the Massive Cluster Survey (MACS). Ten of these systems are new discoveries; only two (MACS J0018.5+1626, aka Cl 0016+1609, and MACS J0454.1-0300, aka MS 0451.6-0305) were previously known. We provide fundamental cluster properties derived from our optical and X-ray follow-up observations as well as the selection function in tabulated form to facilitate cosmological studies using this sample.

The X-ray brightest clusters of galaxies from the Massive Cluster Survey

We present a statistically complete sample of very X-ray luminous galaxy clusters detected in the MAssive Cluster Survey (MACS). This second MACS release comprises all 34 MACS clusters with nominal X-ray fluxes in excess of 2 x 10 -12 erg s -1 cm -2 (0.1-2.4 keV) in the ROSAT Bright Source Catalogue; two-thirds of them are new discoveries. Extending over the redshift range from 0.3 to 0.5, this subset complements the complete sample of the 12 most distant MACS clusters (z > 0.5) published in 2007 and further exemplifies the efficacy of X-ray selection for the compilation of samples of intrinsically massive galaxy clusters. Extensive follow-up observations with Chandra/ACIS led to three additional MACS cluster candidates being eliminated as (predominantly) X-ray point sources. For another four clusters - which, however, remain in our sample of 34 - the point-source contamination was found to be about 50 per cent. The median X-ray luminosity of 1.3 x 10 45 erg s -1 (0.1-2.4 keV, Chandra, within r 500 ) of the clusters in this subsample demonstrates the power of the MACS strategy to find the most extreme and rarest clusters out to significant redshift. A comparison of the optical and X-ray data for all clusters in this release finds a wide range of morphologies with no obvious bias in favour of either relaxed or merging systems.

Diffraction grating transmission efficiencies for XUV and soft x rays

Efficiencies for diffraction of 45-275-eV x rays into orders by interferometrically formed, electrodeposited, gold transmission gratings have been measured on the 4 degrees beam line at the Stanford Synchrotron Radiation Project (SSRP). Anomalous dispersion affects the observed efficiency since the gold is partially transmitting to x rays. Model calculations which include anomalous dispersion are in good agreement with observations. With a suitable choice of material and thickness, a grating can be optimized for a given wavelength range by reducing the zero order transmission and enhancing the higher orders. Even orders are suppressed for a grating with equal slit and wire sizes.

Deep XMM-Newton and Chandra Observations of Cl J1226.9+3332: A Detailed X-Ray Mass Analysis of a z = 0.89 Galaxy Cluster

Deep XMM-Newton and Chandra observations of Cl J1226.9+3332 at z = 0.89 have enabled the most detailed X-ray mass analysis of any such high-redshift galaxy cluster. The XMM-Newton temperature profile of the system shows no sign of central cooling, with a hot core and a radially declining profile. A temperature map shows asymmetry with a hot region that appears to be associated with a subclump of galaxies at the cluster redshift but is not visible in the X-ray surface brightness. This is likely to be the result of a merger event in the cluster but does not appear to significantly affect the overall temperature profile. The XMM-Newton temperature profile and combined Chandra and XMM-Newton emissivity profile allowed precise measurements of the global properties of Cl J1226.9+3332. Within an overdensity radius of 500 times the critical density at z = 0.89 (R500), we find kT = 10.4 ± 0.6 keV, Z = 0.16 ± 0.05 Z☉, and M = 5.2 × 1014 M☉. We obtain profiles of the metallicity, entropy, cooling time, and gas fraction and find a high concentration parameter for the total density profile of the system. The global properties are compared with the local L-T and M-T relations, and we are able to make the first observational test of the predicted evolution of the YX-M500 relation. We find that departures from these scaling relations are most likely caused by an underestimate of the total mass by ~30% in the X-ray hydrostatic mass analysis due to the apparent recent or ongoing merger activity.

High Resolution Imaging X-Ray Detector For Astronomical Measurements

We describe the X-ray detector which will provide second-of-arc images for the HEAO-B X-ray observatory. The instrument uses microchannel plates as a photocathode surface and imaging photoelectron multiplier, and a crossed wire grid as a two-dimensional position-sensitive detector. Position determination is accomplished by electronic interpolation between the coarse grid wires. The detector provides the arrival time and position of each event which occurs with-in its field of view. We describe our measurements of the spatial resolution (15µm), temporal resolution (7.811s), image distortion (<10µm), uniformity of efficiency (< +10%), dark counting rate (0.2 cts cm-2 s-1), and quantum efficiency (10% at 1.54 key) of this detector. We also give a summary of the procedure which we have found useful during the initial turn-on of microchannel plates.