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Register a new userRobust quantitative measures of cluster X-ray morphology, and comparisons between cluster characteristics
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To investigate the relationships between dynamical status and other important characteristics of galaxy clusters, we conducted a study of X-ray cluster morphology using a sample of 101 clusters at redshift z=0.05-1 taken from the Chandra archive. The X-ray morphology is quantitatively characterized by a series of objectively measured simple statistics of the X-ray surface brightness distribution, which are designed to be robust against variations of image quality caused by various exposure times and various cluster redshifts. We found: (1) The distorted and non-distorted clusters occupy well-defined loci in the L-T plane, demonstrating the measurements of the global luminosity and temperature for distorted clusters should be interpreted with caution, or alternatively, a rigorous morphological characterization is necessary when we use a sample of clusters with heterogeneous morphological characteristics to investigate the L-T or other related scaling relations. (2) Ellipticity and Off-center show no evolutionary effects between high and low redshift cluster subsets, while there may be a hint of weak evolutions for the Concentration and Asymmetry, in such a way that high-z clusters show more distorted morphology. (3) No correlation is found between X-ray morphology and X-ray luminosity or X-ray morphology and X-ray temperature of clusters, implying that interaction of clusters may not enhance or decrease the luminosity or temperature of clusters for extended period of time.
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In this work we present combined optical and X-ray cluster detection methods in an area near the North Galactic Pole area, previously covered by the SDSS and 2dF optical surveys. The same area has been covered by shallow ($sim 1.8$ deg$^{2}$) XMM-{em Newton} observations. The optical cluster detection procedure is based on merging two independent selection methods - a smoothing+percolation technique, and a Matched Filter Algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the SAS v.5.2 package. The final optical sample counts 9 candidate clusters with richness of more than 20 galaxies, corresponding roughly to APM richness class. Three, of our optically detected clusters are also detected in our X-ray survey.
Sensitive, wide-area X-ray surveys which would be possible with the WFXT will detect huge samples of virialized objects spanning the mass range from sub-groups to the most massive clusters, and extending in redshift to beyond z=2. These samples will be an excellent dataset for carrying out many traditional cosmological tests using the cluster mass function and power spectrum. Uniquely, WFXT will be able not only to detect clusters but also to make detailed X-ray measurements for a large number of clusters and groups right from the survey data. Very high quality measurements of the cluster mass function and spatial correlation over a very wide range of masses, spatial scales, and redshifts, will be useful for expanding the cosmological discovery space, and in particular, in searching for departures from the concordant Lambda-CDM cosmological model. Finding such departures would have far-reaching implications on our understanding of the fundamental physics which governs the Universe.
The global structure of galaxy clusters and its evolution are tested within a large set of TREESPH simulations, so to allow a fair statistical comparison with available X-ray data. Structure tests are based on the power ratios, introduced by Buote & Tsai. Cosmological models considered are CDM, LCDM (Omega_L=0.7) and CHDM (1 mass.neu., Omega_h = 0.2). All models are normalized to provide a fair number density of clusters. For each model we run a P3M simulation in a large box, where we select the most massive 40 clusters. Going back to the initial redshift we run a hydro-TREESPH simulation for each of them. In this way we perform a statistical comparison of the global morphology of clusters, for each cosmological model, with ROSAT data, using Student t-test, F-test and K-S test. The last test and its generalization to 2--D distributions are also used to compare the joint distributions of 2 or 3 power ratios. We find that, using DM distribution, instead of gas, as done by some authors, leads to biased results, as baryons are distributed in a less structured way than DM. We also find that the cosmological models considered have different behaviours in these tests: LCDM has the worst performance. CDM and our CHDM have similar scores. The general trend of power ratio distributions is already fit by these models, but a further improvement is expected either from a different DM mix or a non-flat CDM model.
We study the distribution of projected offsets between the cluster X-ray centroid and the brightest cluster galaxy (BCG) for 65 X-ray selected clusters from the Local Cluster Substructure Survey (LoCuSS), with a median redshift of z=0.23. We find a clear correlation between X-ray/BCG projected offset and the logarithmic slope of the cluster gas density profile at 0.04r500 (alpha), implying that more dynamically disturbed clusters have weaker cool cores. Furthermore, there is a close correspondence between the activity of the BCG, in terms of detected H_alpha and radio emission, and the X-ray/BCG offset, with the line emitting galaxies all residing in clusters with X-ray/BCG offsets of <~15 kpc. Of the BCGs with alpha < -0.85 and an offset < 0.02r500, 96 per cent (23/24) have optical emission and 88 per cent (21/24) are radio active, while none has optical emission outside these criteria. We also study the cluster gas fraction (fgas) within r500 and find a significant correlation with X-ray/BCG projected offset. The mean fgas of the `small offset clusters (< 0.02r500) is 0.106+/-0.005 (sigma=0.03) compared to 0.145+/-0.009 (sigma=0.04) for those with an offset > 0.02r500, indicating that the total mass may be systematically underestimated in clusters with larger X-ray/BCG offsets. Our results imply a link between cool core strength and cluster dynamical state consistent with the view that cluster mergers can significantly perturb cool cores, and set new constraints on models of the evolution of the intracluster medium.
We present X-ray spectral and timing analysis of members of the young open cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members, or probable members, of the cluster. Stars of all spectral types have been detected, from the early-types to the late-M dwarfs. Despite the capability of the instrument to recognize up to 3 thermal components, the X-ray spectra of the G, K and M members of the cluster are well described with two thermal components (at kT$_1 sim$ 0.3-0.5 keV and kT$_2 sim$ 1.0-1.2 keV respectively) while the X-ray spectra of F members require only a softer 1-T model. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that approximately 46% of the members of IC 2391 are variable with a confidence level $>$99%. The comparison of our data with those obtained with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no evidence of significant variability on these time scales, suggesting that long-term variations due to activity cycles similar to that on the Sun are not common, if present at all, among these young stars.
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