M. Arnaud

Galaxy cluster X-ray luminosity scaling relations from a representative local sample (REXCESS)

We examine the X-ray luminosity scaling relations of 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS). The objects are selected only in X-ray luminosity, optimally sampling the cluster luminosity function. Temperatures range from 2 to 9 keV, and there is no bias toward any particular morphological type. To reduce measurement scatter we extract pertinent values in an aperture corresponding to R500, estimated using the tight correlation between YX (the product of gas mass and temperature) and total mass. The data exhibit power law relations between bolometric X-ray luminosity and temperature, YX and total mass, all with slopes that are significantly steeper than self-similar expectations. We examine the possible causes for the steepening, finding that structural variations have little effect and that the primary driver appears to be a systematic variation of the gas content with mass. Scatter about the relations is dominated in all cases by the presence of cool cores. The natural logarithmic scatter about the raw X-ray luminosity-temperature relation is about 70 per cent, and about the X-ray luminosity-YX relation it is 40 per cent. Systems with more morphological substructure show similar scatter about scaling relations than clusters with less substructure, due to the preponderance of cool core systems in the regular cluster subsample. Cool core and morphologically disturbed systems occupy distinct regions in the residual space with respect to the best fitting mean relation, the former lying systematically at the high luminosity side, the latter lying systematically at the low luminosity side. Simple exclusion of the central regions serves to reduce the scatter about the scaling relations by more than a factor of two. The scatter reduces by a similar amount with the use of the central gas density as a third parameter. Using YX as a total mass proxy, we derive a Malmquist bias-corrected local luminosity-mass relation and compare with other recent determinations. Our results indicate that luminosity can be a reliable mass proxy with controllable scatter, which has important implications for upcoming all-sky cluster surveys, such as those to be undertaken with Planck and eROSITA ,a nd ultimately for the use of the cluster population for cosmological purposes.

The MCXC: a meta-catalogue of x-ray detected clusters of galaxies

We present the compilation and properties of a meta-catalogue of X-ray detected clusters of galaxies, the MCXC. This very large catalogue is based on publicly available ROSAT All Sky Survey-based (NORAS, REFLEX, BCS, SGP, NEP, MACS, and CIZA) and serendipitous (160SD, 400SD, SHARC, WARPS, and EMSS) cluster catalogues. Data have been systematically homogenised to an overdensity of 500, and duplicate entries from overlaps between the survey areas of the individual input catalogues are carefully handled. The MCXC comprises 1743 clusters with virtually no duplicate entries. For each cluster the MCXC provides three identifiers, a redshift, coordinates, membership in the original catalogue, and standardised 0.1−2.4 keV band luminosity L500 ,t otal massM500, and radius R500. The meta-catalogue additionally furnishes information on overlaps between the input catalogues and the luminosity ratios when measurements from different surveys are available, and gives notes on individual objects. The MCXC is available in electronic format for maximum usefulness in X-ray, SZ, and other multiwavelength studies.

Gas entropy in a representative sample of nearby X-ray galaxy clusters (REXCESS): Relationship to gas mass fraction

We examine the radial entropy distribution and its scaling using 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS), a sample in the temperature range 2-9 keV selected in X-ray luminosity only, with no bias toward any particular morphological type. The entropy profi les are robustly measured at least out to R1000 in all systems and out to R500 in thirteen systems. Compared to theoretical expectations from non-radiative cosmological simulations, the observed distributions show a radial and mass-dependent excess entropy, such that the excess is greater and extends to larger radii in lower mass systems. At R500, the mass dependence and entropy excess are both negligible within the large observational and theoretical uncertaint ies. Mirroring this behaviour, the scaling of gas entropy is shal lower than self-similar in the inner regions, but steepens w ith radius, becoming consistent with self-similar at R500. There is a large dispersion in scaled entropy in the inner re gions, apparently linked to the presence of cool cores and dynamical activity; at larger radii the dispersion decreases by approximately a factor of two to 30 per cent, and the dichotomy between subsamples disappears. There are two peaks in the distribution of both inner slope and, after parameterising the profiles with a power law plus const ant model, in central entropy K0. However, we are unable to distinguish between a bimodal or a left-skewed distribution of K0 with the present data. The distribution of outer slopes is un imodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles b y the gas mass fraction profile fgas(< R), leads to a remarkable reduction in the scatter, implying t hat gas mass fraction variations with radius and mass are the cause of the observed entropy structural and scaling properties. The results are consistent with the picture of a cluster population in which entropy modification is cent rally concentrated and extends to larger radii at lower mass, leading to both a radial and a mass-dependence in the gas mass fraction, but which is increasingly self-similar at large radius. T he observed normalisation, however, would suggest entropy modificatio n at least up to R1000, and even beyond, in all but the most massive systems. We discuss a tentative scenario to explain the observed behaviour of the entropy and gas mass fraction in the REXCESS sample, in which a combination of extra heating and merger mixing maintains an elevated central entropy level in the majority of the population, and a smaller fraction of systems is able to develop a cool core.

Calibration of the galaxy cluster M-500-Υ-x relation with XMM-Newton

The quantity YX, the product of the X-ray temperature TX and gas mass Mg, has recently been proposed as a robust low-scatter mass indicator for galaxy clusters. Using precise measurements from XMM-Newton data of a sample of 10 relaxed nearby clusters, spanning a YX range of 10 13 -10 15 MkeV, we investigate the M500-YX relation. The M500-YX data exhibit a power law relation with slope α = 0.548 ± 0.027, close to the self-similar value (3/5) and independent of the mass range considered. However, the normalisation is ∼20% below the prediction from numerical simulations including cooling and galaxy feedback. We discuss two effects that could contribute to the normalisation offset: an underestimate of the true mass due to the hydrostatic equilibrium assumption used in X-ray mass estimates, and an underestimate of the hot gas mass fraction in the simulations. A comparison of the functional form and scatter of the relations between various observables and the mass suggest that YX may indeed be a better mass proxy than TX or Mg,500.

Galaxy-cluster gas-density distributions of the representative XMM-Newton cluster structure survey (REXCESS)

We present a study of the structural and scaling properties of the gas distributions in the intracluster medium (ICM) of 31 nearby (z 3 keV scale self-similarly, with no temperature dependence of gas-density normalisation. The REXCESS sample allows us to investigate the correlations between cluster properties and dynamical state. We find no evidence of correlations between cluster dynamical state and either the gas density slope in the inner regions or temperature, but do find some evidence of a correlation between dynamical state and outer gas density slope. We also find a weak correlation between dynamical state and both central gas normalisation and inner cooling times, but this is only significant at the 10% level. We conclude that, for the X-ray cluster population as a whole, both the central gas properties and the angle-averaged, large-scale gas properties are linked to the cluster dynamical state. We also investigate the central cooling times of the clusters. While the cooling times span a wide range, we find no evidence of a significant bimodality in the distributions of central density, density gradient, or cooling time. Finally, we present the gas mass-temperature relation for the REXCESS sample, finding that h(z)Mgas ∝ T 1.99±0.11 , which is consistent with the expectation of self-similar scaling modified by the presence of an entropy excess in the inner regions of the cluster and consistent with earlier work on relaxed cluster samples. We measure a logarithmic intrinsic scatter in this relation of ∼10%, which should be a good measure of the intrinsic scatter in the Mgas−T relation for the cluster population as a whole.

Substructure of the galaxy clusters in the REXCESS sample: observed statistics and comparison to numerical simulations

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.

Brightest Cluster Galaxies and Core Gas Density in REXCESS Clusters

We investigate the relationship between brightest cluster galaxies (BCGs) and their host clusters using a sample of nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey. The sample was imaged with the Southern Observatory for Astrophysical Research in R band to investigate the mass of the old stellar population. Using a metric radius of 12 h ?1?kpc, we found that the BCG luminosity depends weakly on overall cluster mass as L BCG M 0.18?0.07 cl, consistent with previous work. We found that 90% of the BCGs are located within 0.035 r 500 of the peak of the X-ray emission, including all of the cool core (CC) clusters. We also found an unexpected correlation between the BCG metric luminosity and the core gas density for non-cool-core (non-CC) clusters, following a power law of ne L 2.7?0.4 BCG (where ne is measured at 0.008 r 500). The correlation is not easily explained by star formation (which is weak in non-CC clusters) or overall cluster mass (which is not correlated with core gas density). The trend persists even when the BCG is not located near the peak of the X-ray emission, so proximity is not necessary. We suggest that, for non-CC clusters, this correlation implies that the same process that sets the central entropy of the cluster gas also determines the central stellar density of the BCG, and that this underlying physical process is likely to be mergers.

The dark matter distribution in z ∼0.5 clusters of galaxies I. Determining scaling relations with weak lensing masses

The total mass of clusters of galaxies is a key parameter for studing massive halos. It relates to numerous gravitational and baryonic processes at play in the framework of large-scale structure formation, thus rendering its determination both important and challenging. From a sample of the 11 X-ray bright clusters selected from the EXCPRES sample, we investigate the optical and X-ray properties of clusters with respect to their total mass derived from weak gravitational lensing. From multicolor, wide-field imaging obtained with MegaCam at CFHT, we derive the shear profile of each individual cluster of galaxies. We carefully investigate all systematic sources related to the weak lensing mass determination. The weak lensing masses are then compared to the X-ray masses obtained from the analysis of XMM-Newton observations assuming hydrostatic equilibrium. We find good agreement between the two mass proxies although a few outliers with either perturbed morphology or poor quality data prevent deriving robust mass estimates. The weak lensing mass is also correlated with the optical richness and the total optical luminosity, as well as with the X-ray luminosity, to provide scaling relations within the redshift range 0.4 < z < 0.6. These relations are in good agreement with previous works at lower redshifts. For the LX − M relation we combine our sample with two other cluster and group samples from the literature, thus covering two decades in mass and X-ray luminosity, with a regular and coherent correlation between the two physical quantities.

Testing adiabatic contraction of dark matter in fossil group candidates

We present deep XMM-Newton observations and ESO WFI optical imaging of two X-ray-selected fossil group candidates, RXC J0216.7-4749 and RXC J2315.7-0222. Using the X-ray data, we derive total mass profiles under the hydrostatic equilibrium assumption. The central regions of RXC J0216.7-4749 are found to be dominated by an X-ray bright AGN, and although we derive a mass profile, uncertainties are large and the constraints are significantly weakened due to the presence of the central source. The total mass profile of RXC J2315.7-0222 is of high quality, being measured in fifteen bins from [0.075−0.75] R500 and containing three data points interior to 30 kpc, allowing comprehensive investigation of its properties. We investigate several mass models based on the standard NFW profile or on the Sersic-like model recently suggested by high-resolution N-body simulations. We find that the addition of a stellar component due to the presence of the central galaxy is necessary for a good analytical model fit. In all mass profile models fitted, the mass concentration is not especially high compared to non-fossil systems. In addition, the modification of the dark matter halo by adiabatic contraction slightly improves the fit. However, our result depends critically on the choice of IMF used to convert galaxy luminosity to mass, which leads to a degeneracy between the central slope of the dark matter profile and the normalisation of the stellar component. While we argue on the basis of the range of M∗/LR ratios that lower M∗/LR ratios are preferred on physical grounds and that adiabatic contraction has thus operated in this system, better theoretical and observational convergence on this problem is needed to make further progess.

X-ray observations of clusters of galaxies

X-ray observations of clusters provide key information on the dark matter, on the formation of structures in the Universe, and can be used to constrain the cosmological parameters. I review our current knowledge, with emphasis on recent XMM and Chandra results.