H. Ebeling

Improved constraints on dark energy from Chandra X-ray observations of the largest relaxed galaxy clusters

We present constraints on the mean matter density, � m, dark energy density, � DE, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction (fgas )i n 42 hot (kT > 5 keV), X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range 0.05 < z < 1.1. Using only the fgas data for the six lowest redshift clusters at z < 0.15, for which dark energy has a negligible effect on the measurements, we measurem = 0.28 ± 0.06 (68 per cent confidence limits, using standard priors on the Hubble constant, H0, and mean baryon density, � b h 2 ). Analysing the data for all 42 clusters, employ- ing only weak priors on H0 andb h 2 , we obtain a similar result onm and a detection of the effects of dark energy on the distances to the clusters at ∼99.99 per cent confidence, with � DE = 0.86 ± 0.21 for a non-flatCDM model. The detection of dark energy is comparable in significance to recent type Ia supernovae (SNIa) studies and represents strong, independent evidence for cosmic acceleration. Systematic scatter remains undetected in the fgas data, despite a weighted mean statistical scatter in the distance measurements of only ∼5 per cent. For a flat cosmology with a constant dark energy equation of state, we measurem = 0.28 ± 0.06 and w =− 1.14 ± 0.31. Combining the fgas data with independent constraints from cosmic mi- crowave background and SNIa studies removes the need for priors onb h 2 and H0 and leads to tighter constraints: � m = 0.253 ± 0.021 and w =− 0.98 ± 0.07 for the same constant-w model. Our most general analysis allows the equation of state to evolve with redshift. Marginalizing over possible transition redshifts 0.05 < zt < 1, the combined fgas + CMB + SNIa data set constrains the dark energy equation of state at late and early times to be w0 =− 1.05 ± 0.29 and wet =− 0.83 ± 0.46, respectively, in agreement with the cosmological constant paradigm. Relaxing the assumption of flatness weakens the constraints on the equation of state by only a factor of ∼2. Our analysis includes conservative allowances for systematic uncertainties as- sociated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint onm and powerful constraints on dark energy from the fgas data bode well for future dark energy studies using the next generation of powerful X-ray observatories, such as Constellation-X.

The ROSAT Brightest Cluster Sample – III. Optical spectra of the central cluster galaxies

We present new spectra of dominant galaxies in X-ray selected clusters of galaxies, which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al. 1998), and 18 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X-ray wavebands. 27 per cent of the central dominant galaxies have emission-line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only (NII)��6548,6584 with Hin absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray-selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X- ray centroid is less for the line-emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more H�-luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in Hluminosity. Galaxies with the more luminous line emission (L(H�)> 10 41 ergs 1 ) show a significantly bluer continuum, whereas lower-luminosity and (NII)-only line emitters have continua that differ little from those of non-line emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B-V)� 0.3, and when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of O stars, which also provide sufficient photoionization to produce theobserved Hluminosity. The large number of lower-mass stars relative to the O star population suggests that this anomalous population is due to a series of starbursts in the central galaxy. The lower H�-luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers, or low-level nuclear activity. The line emission from the systems showing only (NII) is very similar to low-level LINER activity commonly found in many normal elliptical galaxies.

The ROSAT Brightest Cluster Sample (BCS): The Cluster X-Ray Luminosity Function within z = 0.3

We present and discuss the X-ray luminosity function (XLF) of the ROSAT Brightest Cluster sample (BCS), an X-ray flux limited sample of clusters of galaxies in the northern hemisphere compiled from ROSAT All-Sky Survey data. The BCS allows the local cluster XLF to be determined with unprecedented accuracy over almost three decades in X-ray luminosity and provides an important reference for searches for cluster evolution at higher redshifts. We find the significance of evolution in both the XLF amplitude and in the characteristic cluster luminosity L -->X to be less than 1.8 ? within the redshift range covered by our sample thereby disproving previous claims of strong evolution within z 0.2.

Truncation of galaxy dark matter halos in high density environments

Aims. Our aim is to constrain the properties of dark matter halos inhabiting high density environments, such as is the case in massive galaxy clusters. Methods. We use galaxy-galaxy lensing techniques that utilize a maximum likelihood method to constrain the parameters of the lenses. It has been demonstrated that such a technique provides strong constraints on the parameters that characterize a galaxy halo, as well as on the aperture mass of these halos. In this analysis, we only use weak shear data and do not include strong lensing constraints. Results. We present the results of a study of galaxy-galaxy lensing in a homogeneous sample of massive x-ray luminous clusters at z ∼ 0.2. These have been observed in three bands with the cfh12k instrument. We find dark matter halos in these clusters to be compact compared to those inferred around isolated field galaxies of equivalent luminosity at this redshift: the half mass radius is found to be smaller than 50 kpc, with a mean total mass of order 0.2× 10 12 M� . This is in good agreement with previous galaxy-galaxy lensing results and with numerical simulations, in particular with the tidal stripping scenario. We thus provide a strong confirmation of tidal truncation from a homogeneous sample of galaxy clusters. Moreover, it is the first time that cluster galaxies are probed successfully using galaxy-galaxy lensing techniques from ground based data.

Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745

Aims. To study at multiple frequencies the radio emission arising from the massive galaxy cluster MACS J0717.5+3745 (z = 0.55). Known to be an extremely complex cluster merger, the system is uniquely suited for an investigation of the phenomena at work in the intra-cluster medium (ICM) during cluster collisions. Methods. We use multi-frequency and multi-resolution data obtained with the Very Large Array radio telescope, and X-ray features revealed by Chandra, to probe the non-thermal and thermal components of the ICM, their relations and interactions. Results. The cluster shows highly complex radio emission. A bright, giant radio halo is detected at frequencies as high as 4.8 GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a radio halo. This radio halo is also the most powerful ever observed, and the second case for which polarized radio emission has been detected, indicating that the magnetic field is ordered on large scales.

Discovery of a Large-Scale Filament Connected to the Massive Galaxy Cluster MACS J0717.5+3745 at z = 0.55* ** ** *

We report the detection of a 4 h Mpc long large-scale filament leading into the massive galaxy cluster MACS J0717.5+3745. The extent of this object well beyond the clusters nominal virial radius (~2.3 Mpc) rules out prior interaction between its constituent galaxies and the cluster and makes it a prime candidate for a genuine filament as opposed to a merger remnant or a double cluster. The structure was discovered as a pronounced overdensity of galaxies selected to have V-R colors close to the cluster red sequence. Extensive spectroscopic follow-up of over 300 of these galaxies in a region covering the filament and the cluster confirms that the entire structure is located at the cluster redshift of z = 0.545. Featuring galaxy surface densities of typically 15 Mpc-2 down to luminosities of 0.13L, the most diffuse parts of the filament are comparable in density to the clumps of red galaxies found around A851 in the only similar study carried out to date (Kodama et al.). Our direct detection of an extended large-scale filament funneling matter onto a massive distant cluster provides a superb target for in-depth studies of the evolution of galaxies in environments of greatly varying density and supports the predictions from theoretical models and numerical simulations of structure formation in a hierarchical picture.

Discovery of radio haloes and double relics in distant MACS galaxy clusters: clues to the efficiency of particle acceleration

We have performed 323 MHz observations with the Giant Metrewave Radio Telescope of the most promising candidates selected from the MACS catalogue. The aim of the work is to extend our knowledge of the radio halo and relic populations to z > 0.3, the epoch in which massive clusters are formed. In MACSJ1149.5+2223 and MACSJ1752.1+4440, we discovered two double-relic systems with a radio halo, and in MACSJ0553.4−3342 we found a radio halo. Archival Very Large Array observations and Westerbork Synthesis Radio Telescope observations have been used to study the polarization and spectral-index properties. The radio halo in MACSJ1149.5+2223 has the steepest spectrum ever found so far in these objects (α ≥ 2). The double relics in MACSJ1149.5+2223 are peculiar in their position that is misaligned with the main merger axis. The relics are polarized up to 30 and 40 per cent in MACSJ1149.5+2223 and MACSJ1752.040+44, respectively. In both cases, the magnetic field is roughly aligned with the relics’ main axes. The spectra in the relics in MACSJ1752.040+44 steepen towards the cluster centre, in agreement with model expectations. X-ray data on MACSJ0553.4−3342 suggest that this cluster is undergoing a major merger, with the merger axis close to the plane of the sky. The cores of the disrupted clusters have just passed each other, but no radio relic is detected in this system. If turbulence is responsible for the radio emission, we argue that it must develop before the core passage. A comparison of double-relic plus halo system with cosmological simulations allows a simultaneous estimate of the acceleration efficiencies at shocks (to produce relics) and of turbulence (to produce the halo).

A CFH12k lensing survey of X-ray luminous galaxy clusters - II. Weak lensing analysis and global correlations

Aims. We present a wide-field multi-color survey of a homogeneous sample of eleven clusters of galaxies for which we measure total masses and mass distributions from weak lensing. This sample, spanning a small range in both X-ray luminosity and redshift, is ideally suited to determining the normalisation of scaling relations between X-ray properties of clusters and their masses (the M − T_X and the M − L_X relations) and also estimating the scatter in these relations at a fixed luminosity. Methods. The eleven clusters in our sample are all X-ray luminous and span a narrow redshift range at z = 0.21 ± 0.04. The weak lensing analysis of the sample is based on ground-based wide-field imaging obtained with the CFH12k camera on CFHT. We use the methodology developed and applied previously on the massive cluster Abell 1689. A Bayesian method, implemented in the Im2shape software, is used to fit the shape parameters of the faint background galaxies and to correct for PSF smearing. A multi-color selection of the background galaxies is applied to retrieve the weak lensing signal, resulting in a background density of sources of ~10 galaxies per square arc minute. With the present data, shear profiles are measured in all clusters out to at least 2 Mpc (more than 15 from the center) with high confidence. The radial shear profiles are fitted with different parametric mass profiles and the virial mass M_(200) is estimated for each cluster and then compared to other physical properties. Results. Scaling relations between mass and optical luminosity indicate an increase of the M/L ratio with luminosity (M/L ∝ L^(0.8)) and a LX−M_(200) relation scaling as L_X ∝ M^(0.83±0.11)_(200) while the normalization of the M_(200) ∝ T^(3/2)_X relation is close to the one expected from hydrodynamical simulations of cluster formation as well as previous X-ray analyses. We suggest that the dispersion in the M_(200) − T_X and M_(200) − L_X relations reflects the different merging and dynamical histories for clusters of similar X-ray luminosities and intrinsic variations in their measured masses. Improved statistics of clusters over a wider mass range are required for a better control of the intrinsic scatter in scaling relations.

A Lensing Survey of X-ray Luminous Galaxy Clusters at Redshift z ~ 0.2: II: CFH12k Weak Lensing Analysis and Global Correlations

Aims. We present a wide-field multi-color survey of a homogeneous sample of eleven clusters of galaxies for which we measure total masses and mass distributions from weak lensing. This sample, spanning a small range in both X-ray luminosity and redshift, is ideally suited to determining the normalisation of scaling relations between X-ray properties of clusters and their masses (the M − T_X and the M − L_X relations) and also estimating the scatter in these relations at a fixed luminosity. Methods. The eleven clusters in our sample are all X-ray luminous and span a narrow redshift range at z = 0.21 ± 0.04. The weak lensing analysis of the sample is based on ground-based wide-field imaging obtained with the CFH12k camera on CFHT. We use the methodology developed and applied previously on the massive cluster Abell 1689. A Bayesian method, implemented in the Im2shape software, is used to fit the shape parameters of the faint background galaxies and to correct for PSF smearing. A multi-color selection of the background galaxies is applied to retrieve the weak lensing signal, resulting in a background density of sources of ~10 galaxies per square arc minute. With the present data, shear profiles are measured in all clusters out to at least 2 Mpc (more than 15 from the center) with high confidence. The radial shear profiles are fitted with different parametric mass profiles and the virial mass M_(200) is estimated for each cluster and then compared to other physical properties. Results. Scaling relations between mass and optical luminosity indicate an increase of the M/L ratio with luminosity (M/L ∝ L^(0.8)) and a LX−M_(200) relation scaling as L_X ∝ M^(0.83±0.11)_(200) while the normalization of the M_(200) ∝ T^(3/2)_X relation is close to the one expected from hydrodynamical simulations of cluster formation as well as previous X-ray analyses. We suggest that the dispersion in the M_(200) − T_X and M_(200) − L_X relations reflects the different merging and dynamical histories for clusters of similar X-ray luminosities and intrinsic variations in their measured masses. Improved statistics of clusters over a wider mass range are required for a better control of the intrinsic scatter in scaling relations.

Strong lensing by a node of the cosmic web - The core of MACS J0717.5+3745 at z = 0.55

We present results of a strong-lensing analysis of MACS J0717.5+3745 (hereafter MACS J0717), an extremely X-ray luminous galaxy cluster at z = 0.55. Observations at different wavelengths reveal a complex and dynamically very active cluster, whose core is connected to a large scale filament extended over several Mpc. Using multi-passband imaging data obtained with the Hubble Space Telescope’s Advanced Camera for Surveys (ACS), we identify 15 multiply imaged systems across the full field of view of ACS, five of which we confirmed spectroscopically in ground-based follow-up observations with the Keck telescope. We use these multiply imaged systems to constrain a parametric model of the mass distribution in the cluster core, employing a new parallelized version of the Lenstool software. The main result is that the most probable description of the mass distribution comprises four clusterscale dark matter haloes. The total mass distribution follows the light distribution but strongly deviates from the distribution of the intra-cluster gas as traced by the X-ray surface brightness. This confirms the complex morphology proposed by previous studies. We interpret this segregation of collisional and collisionless matter as strong evidence of multiple mergers and ongoing dynamical activity. MACS J0717 thus constitutes one of the most disturbed clusters presently known and, featuring a projected mass within the ACS field of view (R = 150” = 960 kpc) of 2.11 ± 0.23 × 10^(15) M_⊙, the system is also one of the most massive known.