We study the substructure statistics of a representative sample of galaxy clusters by means of two currently popular substructure characterisation methods, power ratios and centroid shifts. We use ...the 31 clusters from the REXCESS sample, compiled from the southern ROSAT All-Sky cluster survey (REFLEX) with a morphologically unbiased selection in X-ray luminosity and redshift, all of which have been reobserved with XMM-Newton. The main goals of this work are to study the relationship between cluster morphology and other bulk properties, and the comparison of the morphology statistics between observations and numerical simulations. We investigate the uncertainties of the substructure parameters via newly-developed Monte Carlo methods, and examine the dependence of the results on projection effects (via the viewing angle of simulated clusters), finding that the uncertainties of the parameters can be quite substantial. Thus while the quantification of the dynamical state of individual clusters with these parameters should be treated with extreme caution, these substructure measures provide powerful statistical tools to characterise trends of properties in large cluster samples. The centre shift parameter, w, is found to be more sensitive in general and offers a larger dynamic range than the power ratios. For the REXCESS sample neither the occurence of substructure nor the presence of cool cores depends on cluster mass; however a weak correlation with X-ray luminosity is present, which is interpreted as selection effect. There is a significant anti-correlation between the existence of substantial substructure and cool cores. The simulated clusters show on average larger substructure parameters than the observed clusters, a trend that is traced to the fact that cool regions are more pronounced in the simulated clusters, leading to stronger substructure measures in merging clusters and clusters with offset cores. Moreover, the frequency of cool regions is higher in the simulations than in the observations, implying that the description of the physical processes shaping cluster formation in the simulations requires further improvement.
X-ray observations of galaxy clusters reveal a large range of morphologies with various degrees of disturbance, showing that the assumptions of hydrostatic equilibrium and spherical shape, which are ...used to determine the cluster mass from X-ray data are not always satisfied. It is therefore important for the understanding of cluster properties as well as for cosmological applications to detect and quantify substructure in X-ray images of galaxy clusters. Two promising methods to do so are power ratios and center shifts. Since these estimators can be heavily affected by Poisson noise and X-ray background, we performed an extensive analysis of their statistical properties using a large sample of simulated X-ray observations of clusters from hydrodynamical simulations. We quantify the measurement bias and error in detail and give ranges where morphological analysis is feasible. A new, computationally fast method to correct for the Poisson bias and the X-ray background contribution in power ratio and center shift measurements is presented and tested for typical XMM-Newton observational data sets. We studied the morphology of 121 simulated cluster images and established structure boundaries to divide samples into relaxed, mildly disturbed and disturbed clusters. In addition, we present a new morphology estimator – the peak of the 0.3–1 r500P3/P0 profile to better identify merging clusters. The analysis methods were applied to a sample of 80 galaxy clusters observed with XMM-Newton. We give structure parameters (P3/P0 in r500, w and P3/P0max) for all 80 observed clusters. Using our definition of the P3/P0 (w) substructure boundary, we find 41% (47%) of our observed clusters to be disturbed.
We describe Sunyaev-Zel'dovich (SZ) effect measurements and analysis of the intracluster medium (ICM) pressure profiles of a set of 45 massive galaxy clusters imaged using Bolocam at the Caltech ...Submillimeter Observatory. We deproject the average pressure profile of our sample into 13 logarithmically spaced radial bins between 0.07R sub(500) and 3.5R sub(500), and we find that a generalized Navarro, Frenk, and White (gNFW) profile describes our data with sufficient goodness-of-fit and best-fit parameters (C sub(500), alpha , beta , gamma , P sub(0) = 1.18, 0.86, 3.67, 0.67, 4.29). We use X-ray data to define cool-core and disturbed subsamples of clusters, and we constrain the average pressure profiles of each of these subsamples. We find that, given the precision of our data, the average pressure profiles of disturbed and cool-core clusters are consistent with one another at R > 0.15/Goo, with cool-core systems showing indications of higher pressure at R <, ~ 0.15R sub(500). In addition, for the first time, we place simultaneous constraints on the mass scaling of cluster pressure profiles, their ensemble mean profile, and their radius-dependent intrinsic scatter between 0.1R sub(500) and 2.0R sub(500). The scatter among profiles is minimized at radii between Asymptotically = to0.2R sub(500) and Asymptotically = to0.5R sub(500), with a value of Asymptotically = to20%. These results for the intrinsic scatter are largely consistent with previous analyses, most of which have relied heavily on X-ray derived pressures of clusters at significantly lower masses and redshifts compared to our sample. Therefore, our data provide further evidence that cluster pressure profiles are largely universal with scatter of Asymptotically = to20%-40% about the universal profile over a wide range of masses and redshifts.
We present initial results from our ongoing program to image the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters at 143 GHz using Bolocam; five clusters and one blank field are described in this ...manuscript. The images have a resolution of 58 arcsec and a radius of 6-7 arcmin, which is approximately r 500-2r 500 for these clusters. We effectively high-pass filter our data in order to subtract noise sourced by atmospheric fluctuations, but we are able to obtain unbiased images of the clusters by deconvolving the effects of this filter. The beam-smoothed rms is 10 Delta *mKCMB in these images; with this sensitivity, we are able to detect the SZ signal to beyond r 500 in binned radial profiles. We have fit our images to beta and Nagai models, fixing spherical symmetry or allowing for ellipticity in the plane of the sky, and we find that the best-fit parameter values are in general consistent with those obtained from other X-ray and SZ data. Our data show no clear preference for the Nagai model or the beta model due to the limited spatial dynamic range of our images. However, our data show a definitive preference for elliptical models over spherical models, quantified by an F ratio of 20 for the two models. The weighted mean ellipticity of the five clusters is = 0.27 ? 0.03, consistent with results from X-ray data. Additionally, we obtain model-independent estimates of Y 500, the integrated SZ y-parameter over the cluster face to a radius of r 500, with systematics-dominated uncertainties of 10%. Our Y 500 values, which are free from the biases associated with model-derived Y 500 values, scale with cluster mass in a way that is consistent with both self-similar predictions and expectations of a 10% intrinsic scatter.
We present two non-parametric deprojection methods aimed at recovering the three-dimensional density and temperature profiles of galaxy clusters from spatially resolved thermal Sunyaev–Zeldovich ...(tSZ) and X-ray surface brightness maps, thus avoiding the use of X-ray spectroscopic data. In both methods, the cluster is assumed spherically symmetric and modelled with an onion-skin structure. The first method follows a direct geometrical approach, in which the deprojection is performed independently for the tSZ and X-ray images, and the resulting profiles are then combined in order to extract density and temperature. The second method is based on the maximization of a single joint (tSZ and X-ray) likelihood function. This allows us to simultaneously fit the two signals by following a Monte Carlo Markov Chain (MCMC) approach. These techniques are tested against both an idealized spherical β-model cluster and a set of clusters extracted from cosmological hydrodynamical simulations with and without instrumental noise. In the first case, the quality of reconstruction is excellent and demonstrates that such methods do not suffer from any intrinsic bias. As for the application to simulations, we projected each cluster along the three orthogonal directions defined by the principal axes of the momentum of inertia tensor. This enables us to check any bias in the deprojection associated to the cluster elongation along the line of sight. After averaging over all the three projection directions, we find an overall good reconstruction, with a small (≲10 per cent) overestimate of the gas density profile. This turns into a comparable overestimate of the gas mass within the virial radius, which we ascribe to the presence of residual gas clumping. Apart from this small bias, the reconstruction has an intrinsic scatter of about 5 per cent, which is dominated by gas clumpiness. Cluster elongation along the line of sight biases the deprojected temperature profile upwards at r≲ 0.2rvir and downwards at larger radii. A comparable bias is also found in the deprojected temperature profile. Overall, this turns into a systematic underestimate of the gas mass, up to 10 per cent. We point out that our recovered temperature profiles are much closer to the mass-weighted profiles than those obtained from the X-ray spectroscopic-like temperature. These results confirm the potentiality of combining tSZ and X-ray imaging observations to the study of the thermal structure of the intra-cluster medium out to large cluster-centric distances.
Galaxy clusters exhibit regular scaling relations among their bulk properties. These relations establish vital links between halo mass and cluster observables. Precision cosmology studies that depend ...on these links benefit from a better understanding of scatter in the mass-observable scaling relations. Here, we study the role of merger processes in introducing scatter into the image relation, using a sample of 121 galaxy clusters simulated with radiative cooling and supernova feedback, along with three statistics previously proposed to measure X-ray surface brightness substructure. These are the centroid variation (w), the axial ratio (n), and the power ratios (image and image). We find that in this set of simulated clusters, each substructure measure is correlated with a cluster's departures image and image from the mean image relation, both for emission-weighted temperatures image and for spectroscopic-like temperatures image, in the sense that clusters with more substructure tend to be cooler at a given halo mass. In all cases, a three-parameter fit to the image relation that includes substructure information has less scatter than a two-parameter fit to the basic image relation.
We describe in detail our characterization of the compact radio source population in 140 GHz Bolocam observations of a set of 45 massive galaxy clusters. We use a combination of 1.4 and 30 GHz data ...to select a total of 28 probable cluster-member radio galaxies and also to predict their 140 GHz flux densities. All of these galaxies are steep-spectrum radio sources and they are found preferentially in the cool-core clusters within our sample. Although none of the individual galaxies are robustly detected in the Bolocam data, the ensemble-average flux density at 140 GHz is consistent with, but slightly lower than, the extrapolation from lower frequencies assuming a constant spectral index. This result indicates that radio contamination is not significant compared with current noise levels in 140 GHz images of massive clusters and is in good agreement with the level of radio contamination found in previous results based on lower frequency data or simulations.
We present a method to recover mass profiles of galaxy clusters by combining data on the thermal Sunyaev–Zeldovich (tSZ) and X-ray imaging, thereby avoiding the use of any information on X-ray ...spectroscopy. This method, which represents the development of the geometrical deprojection technique presented in a previous paper by Ameglio et al., implements the solution of the hydrostatic equilibrium equation. In order to quantify the efficiency of our mass reconstructions, we apply our technique to a set of hydrodynamical simulations of galaxy clusters. We propose two versions of our method of mass reconstruction. Method 1 is completely model-independent and assumes the values of gas density and total mass within different radial bins as fitting parameters. Method 2 assumes instead the analytic mass profile proposed by Navarro, Frenk & White (NFW). We find that the main source of bias in recovering the mass profiles is due to deviations from hydrostatic equilibrium, which cause an underestimation of the mass of about 10 per cent at r500 and up to 20 per cent at the virial radius. Method 1 provides a reconstructed mass which is biased low by about 10 per cent, with a 20 per cent scatter, with respect to the true mass profiles. Method 2 proves to be more stable, reducing the scatter to 10 per cent, but with a larger bias of 20 per cent, mainly induced by the deviations from equilibrium in the outskirts. To better understand the results of Method 2, we check how well it allows us to recover the relation between mass and concentration parameter. When analysing the three-dimensional mass profiles, we find that including in the fit the inner 5 per cent of the virial radius biases high the halo concentration, thus suggesting that the NFW profile is not a perfect fit in the central regions of our simulations including cooling and star formation. Also, at a fixed mass, hotter clusters tend to have larger concentration. Our procedure recovers the concentration parameter essentially unbiased but with a scatter of about 50 per cent. In general, our analysis demonstrates that combining X-ray imaging with spatially resolved tSZ data is a valid alternative to using X-ray spectroscopy to recover the mass of galaxy clusters.
The angular-diameter distance DA of a galaxy cluster can be measured by combining its X-ray emission with the cosmic microwave background fluctuation due to the Sunyaev–Zeldovich (SZ) effect. The ...application of this distance indicator usually assumes that the cluster is spherically symmetric, the gas is distributed according to the isothermal β-model, and the X-ray temperature is an unbiased measure of the electron temperature. We test these assumptions with galaxy clusters extracted from an extended set of cosmological N-body/hydrodynamical simulations of a Λ cold dark matter concordance cosmology, which include the effect of radiative cooling, star formation and energy feedback from supernovae. We find that, due to the temperature gradients which are present in the central regions of simulated clusters, the assumption of isothermal gas leads to a significant underestimate of DA. This bias is efficiently corrected by using the polytropic version of the β-model to account for the presence of temperature gradients. In this case, once irregular clusters are removed, the correct value of DA is recovered with a ∼5 per cent accuracy on average, with a ∼20 per cent intrinsic scatter due to cluster asphericity. This result is valid when using either the electron temperature or a spectroscopic-like temperature. Instead When using the emission-weighted definition for the temperature of the simulated clusters, DA is biased low by ∼20 per cent. We discuss the implications of our results for an accurate determination of the Hubble constant H0 and of the density parameter Ωm. We find that, at least in the case of ideal (i.e. noiseless) X-ray and SZ observations extended out to r500, H0 can be potentially recovered with exquisite precision, while the resulting estimate of Ωm, which is unbiased, has typical errors ΔΩm≃ 0.05.
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