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We present a gravitational lensing and X-ray analysis of a massive galaxy cluster and its surroundings. The core of MACS J0717.5+3745 (⁠|$M(R\lt 1\, {\rm Mpc})\sim$| |$2 \times 10^{15}\, \, {\rm M}_{\odot }$|⁠, |$z$| = 0.54) is already known to contain four merging components. We show that this is surrounded by at least seven additional substructures with masses ranging |$3.8{-}6.5\times 10^{13}\, \, {\rm M}_{\odot }$|⁠, at projected radii 1.6–4.9 Mpc. We compare MACS J0717 to mock lensing and X-ray observations of similarly rich clusters in cosmological simulations. The low gas fraction of substructures predicted by simulations turns out to match our observed values of 1– |$4{{\ \rm per\ cent}}$|⁠. Comparing our data to three similar simulated haloes, we infer a typical growth rate and substructure infall velocity. That suggests MACS J0717 could evolve into a system similar to, but more massive than, Abell 2744 by |$z$| = 0.31, and into a ∼ |$10^{16}\, \, {\rm M}_{\odot }$| supercluster by |$z$| = 0. The radial distribution of infalling substructure suggests that merger events are strongly episodic; however, we find that the smooth accretion of surrounding material remains the main source of mass growth even for such massive clusters. [ABSTRACT FROM AUTHOR]

We present the results of a multiwavelength investigation of the very X-ray luminous galaxy cluster MACSJ0553.4-3342 (z=0.4270; hereafter MACSJ0553). Combining high-resolution data obtained with the Hubble Space Telescope and the Chandra X-ray Observatory with ground-based galaxy spectroscopy, our analysis establishes the system unambiguously as a binary, post-collision merger of massive clusters. Key characteristics include perfect alignment of luminous and dark matter for one component, a separation of almost 650 kpc (in projection) between the dark-matter peak of the other subcluster and the second X-ray peak, extremely hot gas (kT > 15 keV) at either end of the merger axis, a potential cold front in the east, an unusually low gas mass fraction of approximately 0.075 for the western component, a velocity dispersion of 1490-130+104 km s-1, and no indication of significant substructure along the line of sight. We propose that the MACSJ0553 merger proceeds not in the plane of the sky, but at a large inclination angle, is observed very close to turnaround, and that the eastern X-ray peak is the cool core of the slightly less massive western component that was fully stripped and captured by the eastern subcluster during the collision. If correct, this hypothesis would make MACSJ0553 a superb target for a competitive study of ram-pressure stripping and the collisional behaviour of luminous and dark matter during cluster formation. [ABSTRACT FROM AUTHOR]

We present a joint optical/X-ray analysis of the massive galaxy cluster Abell 2744 (z = 0.308). Our strong- and weak-lensing analysis within the central region of the cluster, i.e. at R < 1 Mpc from the brightest cluster galaxy, reveals eight substructures, including the main core. All of these dark matter haloes are detected with a significance of at least 5s and feature masses ranging from 0.5 to 1.4 × 1014 M⊙ within R < 150 kpc. Merten et al. and Medezinski et al. substructures are also detected by us. We measure a slightly higher mass for the main core component than reported previously and attribute the discrepancy to the inclusion of our tightly constrained strong-lensing mass model built on Hubble Frontier Fields data. X-ray data obtained by XMM-Newton reveal four remnant cores, one of them a new detection, and three shocks. Unlike Merten et al., we find all cores to have both dark and luminous counterparts. A comparison with clusters of similar mass in the Millennium XXL simulations yields no objects with as many massive substructures as observed in Abell 2744, confirming that Abell 2744 is an extreme system. We stress that these properties still do not constitute a challenge to cold dark matter, as caveats apply to both the simulation and the observations: for instance, the projected mass measurements from gravitational lensing and the limited resolution of the subhaloes finders. We discuss implications of Abell 2744 for the plausibility of different dark matter candidates and, finally, measure a new upper limit on the self-interaction cross-section of dark matter of sDM < 1.28 cm² g-1 (68 per cent CL), in good agreement with previous results from Harvey et al. [ABSTRACT FROM AUTHOR]

We present a high-precision mass model of the galaxy cluster MACS J1149.6+ 2223, based on a strong gravitational lensing analysis of Hubble Space Telescope Frontier Fields (HFF) imaging data and spectroscopic follow-up with Gemini/Gemini Multi-Object Spectrographs (GMOS) and Very Large Telescope (VLT)/Multi Unit Spectroscopic Explorer (MUSE). Our model includes 12 new multiply imaged galaxies, bringing the total to 22, composed of 65 individual lensed images. Unlike the first two HFF clusters, Abell 2744 and MACS J0416.1-2403, MACS J1149 does not reveal as many multiple images in the HFF data. Using the LENSTOOL software package and the new sets of multiple images, we model the cluster with several cluster-scale dark matter haloes and additional galaxy-scale haloes for the cluster members. Consistent with previous analyses, we find the system to be complex, composed of five cluster-scale haloes. Their spatial distribution and lower mass, however, makes MACS J1149 a less powerful lens. Our best-fitting model predicts image positions with an rms of 0.91 arcsec. We measure the total projected mass inside a 200-kpc aperture as (1.840 ± 0.006) × 1014 M⊙, thus reaching again 1 per cent precision, following our previous HFF analyses of MACS J0416.1-2403 and Abell 2744. In light of the discovery of the first resolved quadruply lensed supernova, SN Refsdal, in one of the multiply imaged galaxies identified in MACS J1149, we use our revised mass model to investigate the time delays and predict the rise of the next image between 2015 November and 2016 January. [ABSTRACT FROM AUTHOR]

We present a high-precision mass model of galaxy cluster Abell 2744, based on a strong gravitational-lensing analysis of the Hubble Space Telescope Frontier Fields (HFF) imaging data, which now include both Advanced Camera for Surveys and Wide Field Camera 3 observations to the final depth. Taking advantage of the unprecedented depth of the visible and near-infrared data, we identify 34 new multiply imaged galaxies, bringing the total to 61, comprising 181 individual lensed images. In the process, we correct previous erroneous identifications and positions of multiple systems in the northern part of the cluster core. With the lenstool software and the new sets of multiple images, we model the cluster using two cluster-scale dark matter haloes plus galaxy-scale haloes for the cluster members. Our best-fitting model predicts image positions with an rms error of 0.79 arcsec, which constitutes an improvement by almost a factor of 2 over previous parametric models of this cluster. We measure the total projected mass inside a 200 kpc aperture as (2.162 ± 0.005) × 1014Mʄ, thus reaching 1 percent level precision for the second time, following the recent HFF measurement of MACSJ0416.1-2403. Importantly, the higher quality of the mass model translates into an overall improvement by a factor of 4 of the derived magnification factor. Together with our previous HFF gravitational lensing analysis, this work demonstrates that the HFF data enables high-precision mass measurements for massive galaxy clusters and the derivation of robust magnification maps to probe the early Universe. [ABSTRACT FROM AUTHOR]

We present a high-precision mass model of the galaxy cluster MACSJ0416.1-2403, based on a strong-gravitational-lensing analysis of the recently acquired Hubble Space Telescope Frontier Fields (HFF) imaging data. Taking advantage of the unprecedented depth provided by HST/Advanced Camera for Survey observations in three passbands, we identify 51 new multiply imaged galaxies, quadrupling the previous census and bringing the grand total to 68, comprising 194 individual lensed images. Having selected a subset of the 57 most securely identified multiply imaged galaxies, we use the lenstool software package to constrain a lens model comprised of two cluster-scale dark-matter haloes and 98 galaxy-scale haloes. Our best-fitting model predicts image positions with an rms error of 0.68 arcsec, which constitutes an improvement of almost a factor of 2 over previous, pre-HFF models of this cluster. We find the total projected mass inside a 200 kpc aperture to be (1.60 ± 0.01) × 1014 M⊙, a measurement that offers a three-fold improvement in precision, reaching the per cent level for the first time in any cluster. Finally, we quantify the increase in precision of the derived gravitational magnification of high-redshift galaxies and find an improvement by a factor of ∼2.5 in the statistical uncertainty. Our findings impressively confirm that HFF imaging has indeed opened the domain of high-precision mass measurements for massive clusters of galaxies. [ABSTRACT FROM PUBLISHER]

We present results of a strong-lensing analysis ofMACS J0717.5+3745 (hereafterMACS 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 2.11×0.23×1015 M☉, the system is also one of the most massive known. [ABSTRACT FROM AUTHOR]

We present results of an extensive morphological, spectroscopic and photometric study of the galaxy population of MACS J 1225 ( ), a major cluster merger with clear segregation of dark and luminous matter, to examine the impact of mergers on galaxy evolution. Based on 436 galaxy spectra obtained with Keck DEep Imaging Multi-Object Spectrograph (DEIMOS), we identified 212 cluster members within 4 Mpc of the cluster centre, and classified them using three spectroscopic types; we find 111 absorption line, 90 emission line [including 23 e(a) and 11 e(b)] and six E+A galaxies. The fraction of absorption(emission)-line galaxies is a monotonically increasing(decreasing) function of both projected galaxy density and radial distance to the cluster centre. More importantly, the six observed E+A cluster members are all located between the dark matter peaks of the cluster and within ∼0.3 Mpc radius of the X-ray flux peak, unlike the E+A galaxies in other intermediate-redshift clusters which are usually found to avoid the core region. In addition, we use Hubble Space Telescope imaging to classify cluster members according to morphological type. We find the global fraction of spiral and lenticular galaxies in MACS J0025 to be among the highest observed to date in clusters at . The observed E+A galaxies are found to be of lenticular type with Sersic indices of ∼2, boosting the local fraction of S0 to 70 per cent between the dark matter peaks. Combing the results of our analysis of the spatial distribution, morphology and spectroscopic features of the galaxy population, we propose that the starburst phase of these E+A galaxies was both initiated and terminated during the first core passage about 0.5–1 Gyr ago, and that their morphology has already been transformed into S0 due to ram pressure and/or tidal forces near the cluster core. By contrast, ongoing starbursts are observed predominantly in infalling galaxies, and thus appears to be unrelated to the cluster merger. [ABSTRACT FROM AUTHOR]

We present constraints on the mean matter density, Ωm, dark energy density, ΩDE, and the dark energy equation of state parameter, w, using Chandra measurements of the X-ray gas mass fraction ( fgas) in 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 measure (68 per cent confidence limits, using standard priors on the Hubble constant, H0, and mean baryon density, ). Analysing the data for all 42 clusters, employing only weak priors on H0 and , we obtain a similar result on Ωm and a detection of the effects of dark energy on the distances to the clusters at ∼99.99 per cent confidence, with for a non-flat ΛCDM 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 measure and w=−1.14 ± 0.31. Combining the fgas data with independent constraints from cosmic microwave background and SNIa studies removes the need for priors on and H0 and leads to tighter constraints: 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 and , 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 associated with instrument calibration, cluster physics and data modelling. The measured small systematic scatter, tight constraint on Ωm 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. [ABSTRACT FROM AUTHOR]

We model the mass distribution in the recently discovered Einstein ring LBG J213512.73−010143 (the ‘Cosmic Eye’) using archival Hubble Space Telescope imaging. We reconstruct the mass density profile of the z= 0.73 lens and the surface brightness distribution of the z= 3.07 source and find that the observed ring is best fitted with a dual-component lens model consisting of a baryonic Sersic component nested within a dark matter halo. The dark matter halo has an inner slope of 1.42+0.24−0.22, consistent with cold dark matter simulations after allowing for baryon contraction. The baryonic component has a mass-to-light ratio of which when evolved to the present day is in agreement with local ellipticals. Within the Einstein radius of 0.77 arcsec (5.6 kpc), the baryons account for 46 ± 11 per cent of the projected lens mass. External shear from a nearby foreground cluster is accurately predicted by the model. The reconstructed surface brightness distribution in the source plane clearly shows two peaks. Through a generalization of our lens inversion method, we conclude that the redshifts of both peaks are consistent with each other, suggesting that we are seeing structure within a single galaxy. [ABSTRACT FROM AUTHOR]

We present constraints on the mean dark energy density, ΩX and dark energy equation of state parameter, wX, based on Chandra measurements of the X-ray gas mass fraction in 26 X-ray luminous, dynamically relaxed galaxy clusters spanning the redshift range . Under the assumption that the X-ray gas mass fraction measured within r2500 is constant with redshift and using only weak priors on the Hubble constant and mean baryon density of the Universe, we obtain a clear detection of the effects of dark energy on the distances to the clusters, confirming (at comparable significance) previous results from Type Ia supernovae studies. For a standard Λ cold dark matter (CDM) cosmology with the curvature ΩK included as a free parameter, we find (68 per cent confidence limits). We also examine extended XCDM dark energy models. Combining the Chandra data with independent constraints from cosmic microwave background experiments, we find and . Imposing the prior constraint , the same data require at 95 per cent confidence. Similar results on the mean matter density and dark energy equation of state parameter, and , are obtained by replacing the cosmic microwave background data with standard priors on the Hubble constant and mean baryon density and assuming a flat geometry. [ABSTRACT FROM AUTHOR]

A detailed X-ray analysis of an XMM–Newton observation of the high-redshift galaxy cluster ClJ1226.9+3332 is presented. After careful consideration of background subtraction issues, the X-ray temperature is found to be 11.5+1.1−0.9 keV, the highest X-ray temperature of any cluster at . The temperature is consistent with the observed velocity dispersion. In contrast to MS 1054−0321, the only other very hot cluster currently known at , features a relaxed X-ray morphology, and its high overall gas temperature is not caused by one or several hotspots. The system thus constitutes a unique example of a high-redshift , high-temperature , relaxed cluster, for which the usual hydrostatic equilibrium assumption and the X-ray mass are most reliable. A temperature profile is constructed (for the first time at this redshift) and is consistent with the cluster being isothermal out to 45 per cent of the virial radius. Within the virial radius (corresponding to a measured overdensity of a factor of 200), a total mass of is derived, with a gas mass fraction of per cent (for a Λ cold dark matter cosmology and ). This total mass is similar to that of the Coma cluster. The bolometric X-ray luminosity is . Analysis of a short Chandra observation confirms the lack of significant point-source contamination, the temperature, and the luminosity, albeit with lower precision. The probabilities of finding a cluster of this mass within the volume of the discovery X-ray survey are for and 0.64 for , making highly unlikely. The entropy profile suggests that entropy evolution is being observed. The metal abundance (of ), gas mass fraction and gas distribution are consistent with those of local clusters; thus the bulk of the metals were in place by . [ABSTRACT FROM AUTHOR]

Using two complementary X-ray galaxy cluster studies we present new cosmological constraints on dark energy. Using Chandra measurements of the X-ray gas mass fraction, fgas, we obtain a detection of the effects of dark energy comparable in significance to recent type Ia supernovae (SNIa) studies. Using X-ray luminosity function (XLF) data from galaxy cluster surveys, we obtain the first interesting determination of the dark energy equation of state from measurements of the growth of cosmic structure in clusters. Both of our experiments provide strong, independent support to the ΛCDM paradigm. [ABSTRACT FROM AUTHOR]