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.

Chandra observations of large samples of galaxy clusters detected in X-rays by ROSAT provide a new, robust determination of the cluster mass functions at low and high redshifts. Statistical and systematic errors are now sufficiently small, and the redshift leverage sufficiently large for the mass function evolution to be used as a useful growth of a structure-based dark energy probe. In this paper, we present cosmological parameter constraints obtained from Chandra observations of 37 clusters with z = 0.55 derived from 400 deg 2 ROSAT serendipitous survey and 49 brightest z ≈ 0.05 clusters detected in the All-Sky Survey. Evolution of the mass function between these redshifts requires Ω Λ > 0 with a ∼ 5σ significance, and constrains the dark energy equationof-state parameter to w 0 = −1.14 ± 0.21, assuming a constant w and a flat universe. Cluster information also significantly improves constraints when combined with other methods. Fitting our cluster data jointly with the latest supernovae, Wilkinson Microwave Anisotropy Probe, and baryonic acoustic oscillation measurements, we obtain w 0 = −0.991 ± 0.045 (stat) ±0.039 (sys), a factor of 1.5 reduction in statistical uncertainties, and nearly a factor of 2 improvement in systematics compared with constraints that can be obtained without clusters. The joint analysis of these four data sets puts a conservative upper limit on the masses of light neutrinos m ν < 0.33 eV at 95% CL. We also present updated measurements of Ω M h and σ 8 from the low-redshift cluster mass function.

We present a catalog of galaxy clusters selected via their Sunyaev-Zel'dovich (SZ) effect signature from 2500 deg 2 of South Pole Telescope (SPT) data. This work represents the complete sample of clusters detected at high significance in the 2500 deg 2 SPT-SZ survey, which was completed in 2011. A total of 677 (409) cluster candidates are identified above a signal-to-noise threshold of ξ = 4.5 (5.0). Ground-and space-based optical and near-infrared (NIR) imaging confirms overdensities of similarly colored galaxies in the direction of 516 (or 76%) of the ξ > 4.5 candidates and 387 (or 95%) of the ξ > 5 candidates; the measured purity is consistent with expectations from simulations. Of these confirmed clusters, 415 were first identified in SPT data, including 251 new discoveries reported in this work. We estimate photometric redshifts for all candidates with identified optical and/or NIR counterparts; we additionally report redshifts derived from spectroscopic observations for 141 of these systems. The mass threshold of the catalog is roughly independent of redshift above z ∼ 0.25 leading to a sample of massive clusters that extends to high redshift. The median mass of the sample is M 500c (ρ crit ) ∼ 3.5 × 10 14 M h −1 70 , the median redshift is z med = 0.55, and the highest-redshift systems are at z >1.4. The combination of large redshift extent, clean selection, and high typical mass makes this cluster sample of particular interest for cosmological analyses and studies of cluster formation and evolution.

The Chandra image of the merging, hot galaxy cluster 1E 0657Ϫ56 reveals a bow shock propagating in front of a bullet-like gas cloud just exiting the disrupted cluster core. This is the first clear example of a shock front in a cluster. From the jumps in the gas density and temperature at the shock, the Mach number of the bulletlike cloud is 2-3. This corresponds to a velocity of 3000-4000 km s relative to the main cluster, which means Ϫ1 that the cloud traversed the core just 0.1-0.2 Gyr ago. The 6-7 keV "bullet" appears to be a remnant of a dense cooling flow region once located at the center of a merging subcluster whose outer gas has been stripped by ram pressure. The bullet's shape indicates that it is near the final stage of being destroyed by ram pressure and gasdynamic instabilities, as the subcluster galaxies move well ahead of the cool gas. The unique simplicity of the shock front and bullet geometry in 1E 0657Ϫ56 may allow a number of interesting future measurements. The cluster's average temperature is 14-15 keV but shows large spatial variations. The hottest gas ( keV) T 1 20 lies in the region of the radio halo enhancement and extensive merging activity involving subclusters other than the bullet.

We present a set of low resolution empirical SED templates for AGNs and galaxies in the wavelength range from 0.03 to 30µm based on the multiwavelength photometric observations of the NOAO Deep-Wide Field Survey Boötes field and the spectroscopic observations of the AGN and Galaxy Evolution Survey. Our training sample is comprised of 14448 galaxies in the redshift range 0 z 1 and 5347 likely AGNs in the range 0 z 5.58. The galaxy templates correspond to the SED templates presented by Assef et al. (2008) extended into the UV and mid-IR by the addition of FUV and NUV GALEX and MIPS 24µm data for the field. We use our templates to determine photometric redshifts for galaxies and AGNs. While they are relatively accurate for galaxies (σ z /(1 + z) = 0.04, with 5% outlier rejection), their accuracies for AGNs are a strong function of the luminosity ratio between the AGN and galaxy components. Somewhat surprisingly, the relative luminosities of the AGN and its host are well determined even when the photometric redshift is significantly in error.We also use our templates to study the mid-IR AGN selection criteria developed by Stern et al. (2005) and Lacy et al. (2004). We find that the Stern et al. (2005) criteria suffers from significant incompleteness when there is a strong host galaxy component and at z ≃ 4.5, when the broad Hα emission line is redshifted into the [3.6] band, but that it is little contaminated by low and intermediate redshift galaxies. The Lacy et al. (2004) criterion is not affected by incompleteness at z ≃ 4.5 and is somewhat less affected by strong galaxy host components, but is heavily contaminated by low redshift star forming galaxies. Finally, we use our templates to predict the color-color distribution of sources in the upcoming WISE mission and define a color criterion to select AGNs analogous to those developed for IRAC photometry. We estimate that in between 640, 000 and 1, 700, 000 AGNs will be identified by these criteria, but without additional information, WISE-selected quasars will have serious completeness problems for z 3.4.

In this paper we present an X-ray image deprojection analysis of EINSTEIN OBSERVATORY imaging data on 207 clusters of galaxies. The resulting radial profiles for luminosity, temperature, and electron density variations are determined from the cluster surface-brightness profiles according to gravitational potential constraints from average X-ray temperatures and optical velocity dispersions. This enables us to determine cooling-flow and other cluster properties, such as baryon fractions, Sunyaev-Zeldovich microwave decrements, and Thomson depths. From the results, we have compiled a catalogue of the detected cooling flows, and investigated their effects on general cluster properties. To assist in the analysis, we have constructed self-consistent correlations between the cluster X-ray luminosity, temperature, and optical velocity-dispersion, using 'orthogonal distance' regression to account for errors in both dimensions of the data. These fits indicate that, in general, the temperatures of clusters are isothermal and that they have spectral β-values consistent with unity.We find that the X-ray luminosity, temperature, and optical velocity dispersion relations depend significantly on the cooling flow mass-deposition rate, through characteristic differences in the density profiles. Clusters of similar cooling flow mass-deposition rate exhibit self-similar density profiles, with larger cooling flows showing higher central densities. This leads to scatter in the luminosity related correlations within the X-ray luminosity, temperature and optical velocity dispersion plane. The segregation in density also leads to dispersion in other related properties such as 'half-light radii' and baryon fractions. The baryon fraction in the cores of cooling flow clusters appears to be higher, but as the density profiles tend to a similar value at larger radii, irrespective of cooling flow property, so too do the baryon fraction profiles appear to rise to a concordant value of greater than 10 percent at 1 Mpc. Thus, this sample indicates that clusters, as a whole, are inconsistent with primordial nucleosynthesis baryon fraction prediction, for a flat Universe, of 6 percent.

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, shock-like 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.

We present preliminary results of the XMM-Newton 50 ksec observation of the Perseus cluster which provides an unprecedented view of the central 0.5 Mpc region. The projected gas temperature declines smoothly by a factor of 2 from a maximum value of ∼ 7 keV in the outer regions to just above 3 keV at the cluster center. Over this same range, the heavy element abundance rises slowly from 0.4 solar to 0.5 solar as the radius decreases from 14 ′ to 5 ′ , and then rises to a peak of almost 0.7 solar at 1.25 ′ before declining to 0.4 at the center. The global east/west asymmetry of the gas temperature and surface brightness distributions, approximately aligned with the chain of bright galaxies, suggests an ongoing merger, although the modest degree of the observed asymmetry certainly excludes a major merger interpretation. The chain of galaxies probably traces the filament along which accretion has started some time ago and is continuing at the present time. A cold and dense (low entropy) cluster core like Perseus is probably well "protected" against the penetration of the gas of infalling groups and poor clusters whereas in non-cooling core clusters like Coma and A1367, infalling subclusters can penetrate deeply into the core region. In Perseus, gas associated with infalling groups may be stripped completely at the outskirts of the main cluster and only compression waves (shocks) may reach the central regions. We argue, and show supporting simulations, that the passage of such a wave(s) can qualitatively explain the overall horseshoe shaped appearance of the gas temperature map (the hot horseshoe surrounds the colder, low entropy core) as well as other features of the Perseus cluster core. These simulations also show that as compression waves traverse the cluster core, they can induce oscillatory motion of the cluster gas which can generate multiple sharp "edges", on opposite sides or the central galaxy. Gas motions induced by mergers may be a natural way to explain the high frequency of "edges" seen in clusters with cooling cores.