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Parametric modeling of galaxy cluster density profiles from weak lensing observations leads to a mass bias, whose detailed understanding is critical in deriving accurate mass-observable relations for constraining cosmological models. Drawing from existing methods, we develop a robust framework for calculating this mass bias in one-parameter fits to simulations of dark matter halos. We show that our approach has the advantage of being independent of the absolute noise level, so that only the number of halos in a given simulation and the representativeness of the simulated halos for real clusters limit the accuracy of the bias estimation. While we model the bias as a log-normal distribution and the halos with a Navarro-Frenk-White profile, our method can be generalized to any bias distribution and parametric model of the radial mass distribution. We find that the log-normal assumption is not strictly valid in the presence of miscentring of halos. We investigate the use of cluster centers derived from weak lensing in the context of mass bias, and tentatively find that such centroids can yield sensible mass estimates if the convergence peak has a signal-to-noise ratio approximately greater than four. In this context we also find that the standard approach to estimating the positional uncertainty of weak lensing mass peaks using bootstrapping severely underestimates the true positional uncertainty for peaks with low signal-to-noise ratios. Though we determine the mass and redshift dependence of the bias distribution for a few experimental setups, our focus remains providing a general approach to computing such distributions., Comment: 19 pages, 12 fiures, 2 tables, submitted to MNRAS

We demonstrate that deep good-seeing VLT/HAWK-I $K_\mathrm{s}$ images complemented with $g$+$z$-band photometry can yield a sensitivity for weak lensing studies of massive galaxy clusters at redshifts \mbox{$0.7\lesssim z \lesssim 1.1$}, which is almost identical to the sensitivity of HST/ACS mosaics of single-orbit depth. Key reasons for this good performance are the excellent image quality frequently achievable for $K_\mathrm{s}$ imaging from the ground, a highly effective photometric selection of background galaxies, and a galaxy ellipticity dispersion that is noticeably lower than for optically observed high-redshift galaxy samples. Incorporating results from the 3D-HST and UltraVISTA surveys we also obtained a more accurate calibration of the source redshift distribution than previously achieved for similar optical weak lensing data sets. Here we studied the extremely massive galaxy cluster RCS2$J$232727.7$-$020437 (\mbox{$z=0.699$}), combining deep VLT/\mbox{HAWK-I} $K_\mathrm{s}$ images (point spread function with a 0\farcs35 full width at half maximum) with LBT/LBC photometry. The resulting weak lensing mass reconstruction suggests that the cluster consists of a single overdensity, which is detected with a peak significance of $10.1\sigma$. We constrained the cluster mass to \mbox{$M_\mathrm{200c}/(10^{15} \mathrm{M}_\odot) =2.06^{+0.28}_{-0.26}(\mathrm{stat.})\pm 0.12 (\mathrm{sys.})$} assuming a spherical Navarro, Frenk \& White model and simulation-based priors on the concentration, making it one of the most massive galaxy clusters known in the \mbox{$z\gtrsim 0.7$} Universe. We also cross-checked the HAWK-I measurements through an analysis of overlapping HST/ACS images, yielding fully consistent estimates of the lensing signal., Comment: 17 pages, 14 figures, 1 table. Matches the version published in A&A. Includes minor changes to the text and corrections from the language editor

Observations of relaxed, massive and distant clusters can provide important tests of standard cosmological models e.g. using the gas mass fraction. We study the very luminous, high redshift ($z=0.902$) galaxy cluster ClJ120958.9+495352 using XMM-Newton data and measure the temperature profile and cooling time to investigate the dynamical status with respect to the presence of a cool core as well as global cluster properties. We use HST weak lensing data to estimate its total mass and determine the gas mass fraction. We perform a spectral analysis using an XMM-Newton observation of 15ks cleaned exposure time. As the treatment of the background is crucial, we use two different approaches to account for the background emission to verify our results. We account for point-spread-function effects and deproject our results to estimate the gas mass fraction of the cluster. We measure weak lensing galaxy shapes from mosaic HST imaging and select background galaxies photometrically in combination with imaging data from the William Herschel Telescope. The X-ray luminosity of ClJ120958.9+495352 in the 0.1-2.4keV band estimated from our XMM-Newton data is $L_X = (13.4_{-1.0}^{+1.2})\times10^{44}$erg/s and thus it is one of the most X-ray luminous clusters known at similarly high redshift. We find clear indications for the presence of a cool core from the temperature profile and the central cooling time, which is very rare at such high redshifts. Based on the weak lensing analysis we estimate a cluster mass of $M_\mathrm{500}/10^{14}M_\odot=4.4^{+2.2}_{-2.0}(\mathrm{stat.})\pm0.6(\mathrm{sys.})$ and a gas mass fraction of $f_\mathrm{gas,2500} = 0.11_{-0.03}^{+0.06}$ in good agreement with previous findings for high redshift and local clusters., Comment: 9 pages, 7 figures, accepted by A&A

We present constraints on the scaling relations of galaxy cluster X-ray luminosity, temperature and gas mass (and derived quantities) with mass and redshift, employing masses from robust weak gravitational lensing measurements. These are the first such results obtained from an analysis that simultaneously accounts for selection effects and the underlying mass function, and directly incorporates lensing data to constrain total masses. Our constraints on the scaling relations and their intrinsic scatters are in good agreement with previous studies, and reinforce a picture in which departures from self-similar scaling laws are primarily limited to cluster cores. However, the data are beginning to reveal new features that have implications for cluster astrophysics and provide new tests for hydrodynamical simulations. We find a positive correlation in the intrinsic scatters of luminosity and temperature at fixed mass, which is related to the dynamical state of the clusters. While the evolution of the nominal scaling relations over the redshift range $0.0

The abundance of massive galaxy clusters is a powerful probe of departures from General Relativity (GR) on cosmic scales. Despite current stringent constraints placed by stellar and galactic tests, on larger scales alternative theories of gravity such as $f(R)$ can still work as effective theories. Here we present constraints on two popular models of $f(R)$, Hu-Sawicki and "designer", derived from a fully self-consistent analysis of current samples of X-ray selected clusters and accounting for all the covariances between cosmological and astrophysical parameters. Using cluster number counts in combination with recent data from the cosmic microwave background (CMB) and the CMB lensing potential generated by large scale structures, as well as with other cosmological constraints on the background expansion history and its mean matter density, we obtain the upper bounds $\log_{10}|f_{R0}| < 4.79$ and $\log_{10}B_0 < 3.75$ at the 95.4 per cent confidence level, for the Hu-Sawicki (with $n=1$) and designer models, respectively. The robustness of our results derives from high quality cluster growth data for the most massive clusters known out to redshifts $z \sim 0.5$, a tight control of systematic uncertainties including an accurate and precise mass calibration from weak gravitational lensing data, and the use of the full shape of the halo mass function over the mass range of our data., Comment: 11 pages, 1 Table, 3 figures, matches published version on PRD

We employ robust weak gravitational lensing measurements to improve cosmological constraints from measurements of the galaxy cluster mass function and its evolution, using X-ray selected clusters detected in the ROSAT All-Sky Survey. Our lensing analysis constrains the absolute mass scale of such clusters at the 8 per cent level, including both statistical and systematic uncertainties. Combining it with the survey data and X-ray follow-up observations, we find a tight constraint on a combination of the mean matter density and late-time normalization of the matter power spectrum, $\sigma_8(\Omega_m/0.3)^{0.17}=0.81\pm0.03$, with marginalized, one-dimensional constraints of $\Omega_m=0.26\pm0.03$ and $\sigma_8=0.83\pm0.04$. For these two parameters, this represents a factor of two improvement in precision with respect to previous work, primarily due to the reduced systematic uncertainty in the absolute mass calibration provided by the lensing analysis. Our new results are in good agreement with constraints from cosmic microwave background (CMB) data, both WMAP and Planck (plus WMAP polarization), under the assumption of a flat $\Lambda$CDM cosmology with minimal neutrino mass. Consequently, we find no evidence for non-minimal neutrino mass from the combination of cluster data with CMB, supernova and baryon acoustic oscillation measurements, regardless of which all-sky CMB data set is used (and independent of the recent claimed detection of B-modes on degree scales). We also present improved constraints on models of dark energy (both constant and evolving), modifications of gravity, and primordial non-Gaussianity. Assuming flatness, the constraints for a constant dark energy equation of state from the cluster data alone are at the 15 per cent level, improving to $\sim 6$ per cent when the cluster data are combined with other leading probes., Comment: 24 pages, 10 figures, 5 tables. v3: typo in table A1 corrected

We present cosmological constraints from measurements of the gas mass fraction, $f_{gas}$, for massive, dynamically relaxed galaxy clusters. Our data set consists of Chandra observations of 40 such clusters, identified in a comprehensive search of the Chandra archive, as well as high-quality weak gravitational lensing data for a subset of these clusters. Incorporating a robust gravitational lensing calibration of the X-ray mass estimates, and restricting our measurements to the most self-similar and accurately measured regions of clusters, significantly reduces systematic uncertainties compared to previous work. Our data for the first time constrain the intrinsic scatter in $f_{gas}$, $(7.4\pm2.3)$% in a spherical shell at radii 0.8-1.2 $r_{2500}$, consistent with the expected variation in gas depletion and non-thermal pressure for relaxed clusters. From the lowest-redshift data in our sample we obtain a constraint on a combination of the Hubble parameter and cosmic baryon fraction, $h^{3/2}\Omega_b/\Omega_m=0.089\pm0.012$, that is insensitive to the nature of dark energy. Combined with standard priors on $h$ and $\Omega_b h^2$, this provides a tight constraint on the cosmic matter density, $\Omega_m=0.27\pm0.04$, which is similarly insensitive to dark energy. Using the entire cluster sample, extending to $z>1$, we obtain consistent results for $\Omega_m$ and interesting constraints on dark energy: $\Omega_\Lambda=0.65^{+0.17}_{-0.22}$ for non-flat $\Lambda$CDM models, and $w=-0.98\pm0.26$ for flat constant-$w$ models. Our results are both competitive and consistent with those from recent CMB, SNIa and BAO data. We present constraints on models of evolving dark energy from the combination of $f_{gas}$ data with these external data sets, and comment on the possibilities for improved $f_{gas}$ constraints using current and next-generation X-ray observatories and lensing data. (Abridged), Comment: 25 pages, 14 figures, 8 tables. Accepted by MNRAS. Code and data can be downloaded from http://www.slac.stanford.edu/~amantz/work/fgas14/ . v2: minor fix to table 1, updated bibliography

In light of the tension in cosmological constraints reported by the Planck team between their SZ-selected cluster counts and Cosmic Microwave Background (CMB) temperature anisotropies, we compare the Planck cluster mass estimates with robust, weak-lensing mass measurements from the Weighing the Giants (WtG) project. For the 22 clusters in common between the Planck cosmology sample and WtG, we find an overall mass ratio of $\left< M_{Planck}/M_{\rm WtG} \right> = 0.688 \pm 0.072$. Extending the sample to clusters not used in the Planck cosmology analysis yields a consistent value of $\left< M_{Planck}/M_{\rm WtG} \right> = 0.698 \pm 0.062$ from 38 clusters in common. Identifying the weak-lensing masses as proxies for the true cluster mass (on average), these ratios are $\sim 1.6\sigma$ lower than the default mass bias of 0.8 assumed in the Planck cluster analysis. Adopting the WtG weak-lensing-based mass calibration would substantially reduce the tension found between the Planck cluster count cosmology results and those from CMB temperature anisotropies, thereby dispensing of the need for "new physics" such as uncomfortably large neutrino masses (in the context of the measured Planck temperature anisotropies and other data). We also find modest evidence (at 95 per cent confidence) for a mass dependence of the calibration ratio and discuss its potential origin in light of systematic uncertainties in the temperature calibration of the X-ray measurements used to calibrate the Planck cluster masses. Our results exemplify the critical role that robust absolute mass calibration plays in cluster cosmology, and the invaluable role of accurate weak-lensing mass measurements in this regard., Comment: 5 pages, 2 figures

We present improved methods for using stars found in astronomical exposures to calibrate both star and galaxy colors as well as to adjust the instrument flat field. By developing a spectroscopic model for the SDSS stellar locus in color-color space, synthesizing an expected stellar locus, and simultaneously solving for all unknown zeropoints when fitting to the instrumental locus, we increase the calibration accuracy of stellar locus matching. We also use a new combined technique to estimate improved flat-field models for the Subaru SuprimeCam camera, forming `star flats' based on the magnitudes of stars observed in multiple positions or through comparison with available SDSS magnitudes. These techniques yield galaxy magnitudes with reliable color calibration (< 0.01 - 0.02 mag accuracy) that enable us to estimate photometric redshift probability distributions without spectroscopic training samples. We test the accuracy of our photometric redshifts using spectroscopic redshifts z_s for ~5000 galaxies in 27 cluster fields with at least five bands of photometry, as well as galaxies in the COSMOS field, finding sigma((z_p - z_s)/(1 + z_s)) ~ 0.03 for the most probable redshift z_p. We show that the full posterior probability distributions for the redshifts of galaxies with five-band photometry exhibit good agreement with redshifts estimated from thirty-band photometry in the COSMOS field. The growth of shear with increasing distance behind each galaxy cluster shows the expected redshift-distance relation for a flat Lambda-CDM cosmology. Photometric redshifts and calibrated colors are used in subsequent papers to measure the masses of 51 galaxy clusters from their weak gravitational shear. We make our Python code for stellar locus matching available at http://big-macs-calibrate.googlecode.com; the code requires only a catalog and filter functions., Comment: Accepted by MNRAS with only minor revisions. Code available: http://big-macs-calibrate.googlecode.com (v2 latex symbols removed from abstract)

We report weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known. This cluster sample, introduced earlier in this series of papers, spans redshifts 0.15 < z_cl < 0.7, and is well suited to calibrate mass proxies for current cluster cosmology experiments. Cluster masses are measured with a standard `color-cut' lensing method from three-filter photometry of each field. Additionally, for 27 cluster fields with at least five-filter photometry, we measure high-accuracy masses using a new method that exploits all information available in the photometric redshift posterior probability distributions of individual galaxies. Using simulations based on the COSMOS-30 catalog, we demonstrate control of systematic biases in the mean mass of the sample with this method, from photometric redshift biases and associated uncertainties, to better than 3%. In contrast, we show that the use of single-point estimators in place of the full photometric redshift posterior distributions can lead to significant redshift-dependent biases on cluster masses. The performance of our new photometric redshift-based method allows us to calibrate `color-cut` masses for all 51 clusters in the present sample to a total systematic uncertainty of ~7% on the mean mass, a level sufficient to significantly improve current cosmology constraints from galaxy clusters. Our results bode well for future cosmological studies of clusters, potentially reducing the need for exhaustive spectroscopic calibration surveys as compared to other techniques, when deep, multi-filter optical and near-IR imaging surveys are coupled with robust photometric redshift methods., Comment: 25 pages, 14 figures. Accepted after minor revision. Lensing masses (table 4) and other data will be made available at http://www.slac.stanford.edu/~dapple/work/wtg.html

This is the first in a series of papers in which we measure accurate weak-lensing masses for 51 of the most X-ray luminous galaxy clusters known at redshifts 0.15

We report on a deep, multiwavelength study of the galaxy cluster MACS J1931.8-2634 using Chandra X-ray, Subaru optical, and VLA 1.4 GHz radio data. This cluster (z=0.352) harbors one of the most X-ray luminous cool cores yet discovered, with an equivalent mass cooling rate within the central 50 kpc is approximately 700 solar masses/yr. Unique features observed in the central core of MACSJ1931.8-2634 hint to a wealth of past activity that has greatly disrupted the original cool core. We observe a spiral of relatively cool, dense, X-ray emitting gas connected to the cool core, as well as highly elongated intracluster light (ICL) surrounding the cD galaxy. Extended radio emission is observed surrounding the central AGN, elongated in the east-west direction, spatially coincident with X-ray cavities. The power input required to inflate these `bubbles' is estimated from both the X-ray and radio emission to reside between 4 and 14e45 erg/s, putting it among the most powerful jets ever observed. This combination of a powerful AGN outburst and bulk motion of the cool core have resulted in two X-ray bright ridges to form to the north and south of the central AGN at a distance of approximately 25 kpc. The northern ridge has spectral characteristics typical of cool cores and is consistent with being a remnant of the cool core after it was disrupted by the AGN and bulk motions. It is also the site of H-alpha filaments and young stars. The X-ray spectroscopic cooling rate associated with this ridge is approximately 165 solar masses/yr, which agrees with the estimate of the star formation rate from broad-band optical imaging (170 solar masses/yr). MACS J1931.8-2634 appears to harbor one of most profoundly disrupted low entropy cores observed in a cluster, and offers new insights into the survivability of cool cores in the context of hierarchical structure formation., Comment: 19 pages, 15 figures, 5 tables. Accepted by MNRAS for publication September 30 2010

We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACSJ0025.4-1222. As was previously shown by the example of the Bullet Cluster (1E0657-56), such systems are ideal laboratories for detecting isolated dark matter, and distinguishing between cold dark matter (CDM) and other scenarios (e.g. self-interacting dark matter, alternative gravity theories). MACSJ0025.4-1222 consists of two merging subclusters of similar richness at z=0.586. We measure the distribution of X-ray emitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass. The total mass distribution in each of the subclusters is clearly offset (at >4sigma significance) from the peak of the hot X-ray emitting gas (the main baryonic component), but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas (0.09^{+0.07}_{-0.03}) and stars (0.010^{+0.007}_{-0.004}), consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field. Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross-section of sigma/m < 4cm^2/g, re-affirming the results from the Bullet Cluster on the collisionless nature of dark matter., Comment: 10 pages, 6 figures, Accepted for publication in ApJ, no revision

The GRavitational lEnsing Accuracy Testing 2008 (GREAT08) Challenge focuses on a problem that is of crucial importance for future observations in cosmology. The shapes of distant galaxies can be used to determine the properties of dark energy and the nature of gravity, because light from those galaxies is bent by gravity from the intervening dark matter. The observed galaxy images appear distorted, although only slightly, and their shapes must be precisely disentangled from the effects of pixelisation, convolution and noise. The worldwide gravitational lensing community has made significant progress in techniques to measure these distortions via the Shear TEsting Program (STEP). Via STEP, we have run challenges within our own community, and come to recognise that this particular image analysis problem is ideally matched to experts in statistical inference, inverse problems and computational learning. Thus, in order to continue the progress seen in recent years, we are seeking an infusion of new ideas from these communities. This document details the GREAT08 Challenge for potential participants. Please visit http://www.great08challenge.info for the latest information., Comment: Published in at http://dx.doi.org/10.1214/08-AOAS222 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org)