We present new weak lensing observations of 1E0657−558 (z = 0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8σ significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen.

We detect a weak unidentified emission line at E = (3.55−3.57)±0.03 keV in a stacked XMM-Newton spectrum of 73 galaxy clusters spanning a redshift range 0.01 − 0.35. MOS and PN observations independently show the presence of the line at consistent energies. When the full sample is divided into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is seen at > 3σ statistical significance in all three independent MOS spectra and the PN "all others" spectrum. The line is also detected at the same energy in the Chandra ACIS-S and ACIS-I spectra of the Perseus cluster, with a flux consistent with XMM-Newton (however, it is not seen in the ACIS-I spectrum of Virgo). The line is present even if we allow maximum freedom for all the known thermal emission lines. However, it is very weak (with an equivalent width in the full sample of only ∼ 1 eV) and located within 50-110 eV of several known faint lines; the detection is at the limit of the current instrument capabilities and subject to significant modeling uncertainties. On the origin of this line, we argue that there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark matter is in sterile neutrinos with m s = 2E = 7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin 2 (2θ) ≈ 7 × 10 −11 , below the previous upper limits. However, based on the cluster masses and distances, the line in Perseus is much brighter than expected in this model, significantly deviating from other subsamples. This appears to be because of an anomalously bright line at E = 3.62 keV in Perseus, which could be an Ar xvii dielectronic recombination line, although its emissivity would have to be 30 times the expected value and physically difficult to understand. In principle, such an anomaly might explain our line detection in other subsamples as well, though it would stretch the line energy uncertainties. Another alternative is the above anomaly in the Ar line combined with the nearby 3.51 keV K line also exceeding expectation by a factor 10-20. Confirmation with Chandra and Suzaku, and eventually Astro-H, are required to determine the nature of this new line. (APJ HAS THE ABRIDGED ABSTRACT) Γ γ (m s , θ) = 1.38 × 10 −29 s −1 sin 2 2θ 10 −7 m s 1 keV 5 ,(1) where the particle mass m s and the "mixing angle" θ

We compare recent results from X-ray, strong lensing, weak lensing, and optical observations with numerical simulations of the merging galaxy cluster 1E 0657-56. X-ray observations reveal a bullet-like subcluster with a prominent bow shock, which gives an estimate for the merger velocity of 4700 km s −1 , while lensing results show that the positions of the total mass peaks are consistent with the centroids of the collisionless galaxies (and inconsistent with the X-ray brightness peaks). Previous studies, based on older observational datasets, have placed upper limits on the self-interaction cross-section of dark matter per unit mass, σ/m, using simplified analytic techniques. In this work, we take advantage of new, higher-quality observational datasets by running full N-body simulations of 1E 0657-56 that include the effects of self-interacting dark matter, and comparing the results with observations. Furthermore, the recent data allow for a new independent method of constraining σ/m, based on the non-observation of an offset between the bullet subcluster mass peak and galaxy centroid. This new method places an upper limit (68% confidence) of σ/m < 1.25 cm 2 g −1 .If we make the assumption that the subcluster and the main cluster had equal mass-to-light ratios prior to the merger, we derive our most stringent constraint of σ/m < 0.7 cm 2 g −1 , which comes from the consistency of the subcluster's observed mass-to-light ratio with the main cluster's, and with the universal cluster

The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004, 2006a) that the total mass does not trace the baryonic mass.

New Chandra X-ray data and extensive optical spectroscopy, obtained with AAOmega on the 3.9 m Anglo-Australian Telescope, are used to study the complex merger taking place in the galaxy cluster Abell 2744. Combining our spectra with data from the literature provides a catalog of 1237 redshifts for extragalactic objects lying within 15 ′ of the cluster center. From these, we confirm 343 cluster members projected within 3 Mpc of the cluster center. Combining positions and velocities, we identify two major substructures, corresponding to the remnants of two major subclusters. The new data are consistent with a post core passage, major merger taking place along an axis that is tilted well out of the plane of the sky, together with an interloping minor merger. Supporting this interpretation, the new X-ray data reveal enriched, low entropy gas from the core of the approaching, major subcluster, lying ∼ 2 ′ north of the cluster center, and a shock front to the southeast of the previously known bright, compact core associated with the receding subcluster. The X-ray morphology of the compact core is consistent with a Bullet-like cluster viewed from within ∼ 45 • of the merger axis. An X-ray peak ∼ 3 ′ northwest of the cluster center, with an associated cold front to the northeast and a trail of low entropy gas to the south, is interpreted as the remnant of an interloping minor merger taking place roughly in the plane of the sky. We infer approximate paths for the three merging components.

We investigate the form and evolution of the X‐ray luminosity–temperature (LX–kT) relation of a sample of 114 galaxy clusters observed with Chandra at 0.1 < z < 1.3. The clusters were divided into subsamples based on their X‐ray morphology or whether they host strong cool cores. We find that when the core regions are excluded, the most relaxed clusters (or those with the strongest cool cores) follow an LX–kT relation with a slope that agrees well with simple self‐similar expectations. This is supported by an analysis of the gas density profiles of the systems, which shows self‐similar behaviour of the gas profiles of the relaxed clusters outside the core regions. By comparing our data with clusters in the Representative XMM‐Newton Cluster Structure Survey (REXCESS) sample, which extends to lower masses, we find evidence that the self‐similar behaviour of even the most relaxed clusters breaks at around 3.5   keV. By contrast, the LX–kT slopes of the subsamples of unrelaxed systems (or those without strong cool cores) are significantly steeper than the self‐similar model, with lower mass systems appearing less luminous and higher mass systems appearing more luminous than the self‐similar relation. We argue that these results are consistent with a model of non‐gravitational energy input in clusters that combines central heating with entropy enhancements from merger shocks. Such enhancements could extend the impact of central energy input to larger radii in unrelaxed clusters, as suggested by our data. We also examine the evolution of the LX–kT relation, and find that while the data appear inconsistent with simple self‐similar evolution, the differences can be plausibly explained by selection bias, and thus we find no reason to rule out self‐similar evolution. We show that the fraction of cool core clusters in our (non‐representative) sample decreases at z > 0.5 and discuss the effect of this on measurements of the evolution in the LX–kT relation.

X‐ray surface brightness fluctuations in the core (650 × 650 kpc) region of the Coma cluster observed with XMM–Newton and Chandra are analysed using a 2D power spectrum approach. The resulting 2D spectra are converted to 3D power spectra of gas density fluctuations. Our independent analyses of the XMM–Newton and Chandra observations are in excellent agreement and provide the most sensitive measurements of surface brightness and density fluctuations for a hot cluster. We find that the characteristic amplitude of the volume filling density fluctuations relative to the smooth underlying density distribution varies from 7–
10 per cent on scales of ∼500 kpc down to ∼5 per cent on scales of ∼30 kpc. On smaller spatial scales, projection effects smear the density fluctuations by a large factor, precluding strong limits on the fluctuations in 3D. On the largest scales probed (hundreds of kpc), the dominant contributions to the observed fluctuations most likely arise from perturbations of the gravitational potential by the two most massive galaxies in Coma, NGC4874 and NGC4889, and the low‐entropy gas brought to the cluster by an infalling group. Other plausible sources of X‐ray surface brightness fluctuations are discussed, including turbulence, metal abundance variations and unresolved sources. Despite a variety of possible origins for density fluctuations, the gas in the Coma cluster core is remarkably homogeneous on scales from ∼500 to ∼30 kpc.

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.