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, and ultimately for the use of the cluster population for cosmological purposes.
We present results from the analysis of a mosaic of 13 XMM-Newton pointings covering the Virgo Cluster from its centre northwards out to a radius r∼ 1.2 Mpc (∼4°.5), reaching the virial radius and ...beyond. This is the first time that the properties of a modestly sized (M
vir∼ 1.4 × 1014 M⊙, kT∼ 2.3 keV), dynamically young cluster have been studied out to the virial radius. The density profile of the cluster can be described by a surprisingly shallow power-law n
e∝r
−β with index β= 1.21 ± 0.12. In the radial range of 0.3r
vir < r < r
vir, the best-fitting temperature drops by roughly 60 per cent. Within a radius r < 450 kpc, the entropy profile has an approximate power law form K∝r
1.1, as expected for gravitationally collapsed gas in hydrostatic equilibrium. Beyond r∼ 450 kpc, however, the temperature and metallicity drop abruptly, and the entropy profile becomes flatter, staying consistently below the expected value by a factor of 2-2.5. The most likely explanation for the unusually shallow density profile and the flattening of entropy at large radius is clumping in the ICM. Our data provide direct observational evidence that the ICM is enriched by metals all the way to r
200 to at least Z= 0.1 Z⊙.
Non-thermal motions in the intracluster medium (ICM) are believed to play a non-negligible role in the pressure support to the total gravitating mass of galaxy clusters. Future X-ray missions, such ...as ASTRO-H and ATHENA, will eventually allow us to directly detect the signature of these motions from high-resolution spectra of the ICM. In this paper, we present a study on a set of clusters extracted from a cosmological hydrodynamical simulation, devoted to explore the role of non-thermal velocity amplitude in characterizing the cluster state and the relation between observed X-ray properties. In order to reach this goal, we apply the X-ray virtual telescope PHOX to generate synthetic observations of the simulated clusters with both Chandra and ATHENA, the latter used as an example for the performance of very high-resolution X-ray telescopes. From Chandra spectra we extract global properties, e.g. luminosity and temperature, and we accurately estimate the gas velocity dispersion along the line of sight achievable from the broadening of emission lines from heavy ions (e.g. Fe) resolved in ATHENA spectra. Given the good agreement found between simulations (true, intrinsic solution) and mock observations (detectable amplitude of non-thermal velocities), we further extend the analysis to the relation between non-thermal velocity dispersion of the gas and the L
X-T scaling law for the simulated clusters. Interestingly, we find a clear dependence of slope and scatter on the selection criterion for the clusters, based on the level of significance of non-thermal motions. Namely, the scatter in the relation is significantly reduced by the exclusion of the clusters, for which we estimate the highest turbulent velocities. Such velocity diagnostics appears therefore as a promising independent way to identify disturbed clusters, in addition to the commonly used morphological inspection.
Context. A precise understanding of the relations between observable X-ray properties of galaxy clusters and cluster mass is a vital part of the application of X-ray galaxy cluster surveys to test ...cosmological models. An understanding of how these relations evolve with redshift is just emerging from a number of observational data sets. Aims. The current literature provides a diverse and inhomogeneous picture of scaling relation evolution. We attempt to transform these results and the data on recently discovered distant clusters into an updated and consistent framework, and provide an overall view of scaling relation evolution from the combined data sets. Methods. We study in particular the most important scaling relations connecting X-ray luminosity, temperature, and cluster mass (M–T, LX–T, and M–LX) combining 14 published data sets supplemented with recently published data of distant clusters and new results from follow-up observations of the XMM-Newton Distant Cluster Project (XDCP) that adds new leverage to efficiently constrain the scaling relations at high redshift. Results. We find that the evolution of the mass-temperature relation is consistent with the self-similar evolution prediction, while the evolution of X-ray luminosity for a given temperature and mass for a given X-ray luminosity is slower than predicted by simple self-similar models. Our best fit results for the evolution factor E(z)α are α = −1.04 ± 0.07 for the M–T relation, \hbox{$\alpha=-0.23^{+0.12}_{-0.62}$}α=−0.23-0.62+0.12 for the L–T relation, and \hbox{$\alpha=-0.93^{+0.62}_{-0.12}$}α=−0.93-0.12+0.62 for the M–LX relation. We also explore the influence of selection effects on scaling relations and find that selection biases are the most likely reason for apparent inconsistencies between different published data sets. Conclusions. The new results provide the currently most robust calibration of high-redshift cluster mass estimates based on X-ray luminosity and temperature and help us to improve the prediction of the number of clusters to be found in future galaxy cluster X-ray surveys, such as eROSITA. The comparison of evolution results with hydrodynamical cosmological simulations suggests that early preheating of the intracluster medium (ICM) provides the most suitable scenario to explain the observed evolution.
We perform hydrodynamical simulations of minor-merger-induced gas sloshing and the subsequent formation of cold fronts in the Virgo cluster. Comparing to observations, we show for the first time that ...the sloshing scenario can reproduce the radii and the contrasts in X-ray brightness, projected temperature and metallicity across the cold fronts quantitatively. The comparison suggests a third cold front 20 kpc north-west of the Virgo core. We identify several new features typical for sloshing cold fronts: an alternating distribution of cool, metal-enriched X-ray brightness excess regions and warm brightness deficit regions of reduced metallicity; a constant or radially decreasing temperature accompanied by a plateau in metallicity inside the cold fronts; a warm rim outside the cold fronts and a large-scale brightness asymmetry. We can trace these new features not only in Virgo, but also in other clusters exhibiting sloshing cold fronts. By comparing synthetic and real observations, we estimate that the original minor-merger event took place about 1.5 Gyr ago when a subcluster of 1-4 × 1013 M⊙ passed the Virgo core at 100-400 kpc distance, where a smaller mass corresponds to a smaller pericentre distance, and vice versa. From our inferred merger geometry, we derive the current location of the disturbing subcluster to be about 1-2 Mpc east of the Virgo core. A possible candidate is M60. Additionally, we quantify the metal redistribution by sloshing and discuss its importance. We verify that the subcluster required to produce the observed cold fronts could be completely ram-pressure-stripped before reaching the Virgo centre, and discuss the conditions required for this to be achieved. Finally, we demonstrate that the bow shock of a fast galaxy passing the Virgo cluster at ∼400 kpc distance also causes sloshing and leads to very similar cold front structures. The responsible galaxy would be located about 2 Mpc north of the Virgo centre. A possible candidate is M85.
Aims. The hierarchical model of structure formation is a key prediction of the Λ cold dark matter model, which can be tested by studying the large-scale environment and the substructure content of ...massive galaxy clusters. We present here a detailed analysis of the clusters RXC J0225.9-4154, RXC J0528.9-3927, and RXC J2308.3-0211, as part of a sample of massive X-ray luminous clusters located at intermediate redshifts. Methods. We used a multiwavelength analysis, combining WFI photometric observations, VIMOS spectroscopy, and the X-ray surface brightness maps. We investigated the optical morphology of the clusters, we looked for significant counterparts in the residual X-ray emission, and we ran several statistical tests to assess their dynamical state. We correlated the results to define various substructure features, to study their properties, and to quantify their influence on simple dynamical mass estimators. Results. RXC J0225.9-4154 has a bi-modal core, and two massive galaxy groups are located in its immediate surroundings; they are aligned in an elongated structure that is also detected in X-rays at the 1σ level. RXC J0528.9-3927 is located in a poor environment; an X-ray centroid shift and the presence of two central BCGs provide mild evidence for a recent and active dynamical history. RXC J2308.3-0211 has complex central dynamics, and it is found at the core of a superstes-cluster. Conclusions. The complexity of the cluster’s central dynamics reflects the richness of its large-scale environment: RXC J0225 and RXC J2308 present a mass fraction in substructures larger than the typical 5−15%, whereas the isolated cluster RXC J0528 does not have any major substructures within its virial radius. The largest substructures are found in the cluster outskirts. The optical morphology of the clusters correlates with the orientation of their BCG, and with the position of the main axes of accretion.
Context. A robust determination of galaxy cluster mass is crucial to use them as cosmological probes, or to study the physics governing their formation and evolution. Applying various estimators on ...well-defined cluster samples is a mandatory step in characterising their respective systematics. Aims. Our main goal is to compare the results of three dynamical mass estimators to the X-ray hydrostatic values. Here we focus on massive galaxy clusters at intermediate redshifts z ~ 0.3. Methods. We estimated dynamical masses with the virial theorem, the Jeans equation, and the caustic method using wide-field VIMOS spectroscopy; the hydrostatic masses were obtained previously from XMM-Newton observations. We investigated the role of colour selection and the impact of substructures on the dynamical estimators. Results. The Jeans and caustic methods give consistent results, whereas the virial theorem leads to masses ~ 15% larger. The Jeans, caustic, and virial masses are respectively ~ 20%, ~ 30%, and ~ 50% larger than the hydrostatic values. Large scatters of ≳ 50% are mainly due to the two outliers RXC J0014 and RXC J1347; excluding the latter increases the mass ratios by ~ 10%, giving a fractional mass bias significant at ≳ 2σ. We found a correlation between the dynamical-to-hydrostatic mass ratio and two substructure indicators, suggesting a bias in the dynamical measurements. The velocity dispersions of blue galaxies are ~ 15% (~ 25% after removing the substructures) larger than that of the red-sequence galaxies; using the latter leads to dynamical masses ~ 10 − 15% smaller. Discarding the galaxies part of substructures reduces the masses by ~ 15%; the effect is larger for the more massive clusters, owing to a higher level of substructures. After the substructure analysis, the dynamical masses are in perfect agreement with the hydrostatic values and the scatters around the mean ratios are divided by approximately two. The mass bias is no longer significant, even after excluding RXC J1347.
We present relations between X-ray luminosity and velocity dispersion (L − σ), X-ray luminosity and gas mass (L − Mgas), and cluster radius and velocity dispersion (r500 − σ) for 62 galaxy clusters ...in the HIFLUGCS, an X-ray flux-limited sample minimizing bias toward any cluster morphology. Our analysis in total is based on ~1.3 Ms of clean X-ray XMM-Newton data and 13439 cluster member galaxies with redshifts. Cool cores are among the major contributors to the scatter in the L − σ relation. When the cool-core-corrected X-ray luminosity is used the intrinsic scatter decreases to 0.27 dex. Even after the X-ray luminosity is corrected for the cool core, the scatter caused by the presence of cool cores dominates for the low-mass systems. The scatter caused by the non-cool-core clusters does not strongly depend on the mass range, and becomes dominant in the high-mass regime. The observed L − σ relation agrees with the self-similar prediction, matches that of a simulated sample with AGN feedback disregarding six clusters with <45 cluster members with spectroscopic redshifts, and shows a common trend of increasing scatter toward the low-mass end, i.e., systems with σ ≤ 500 kms-1. A comparison of observations with simulations indicates an AGN-feedback-driven impact in the low-mass regime. The best fits to the L − Mgas relations for the disturbed clusters and undisturbed clusters in the observational sample closely match those of the simulated samples with and without AGN feedback, respectively. This suggests that one main cause of the scatter is AGN activity providing feedback in different phases, e.g. during a feedback cycle. The slope and scatter in the observed r500 − σ relation is similar to that of the simulated sample with AGN feedback except for a small offset but still within the scatter.
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 profiles 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 uncertainties. Mirroring this behaviour, the scaling of gas entropy is shallower than self-similar in the inner regions, but steepens with radius, becoming consistent with self-similar at R500. There is a large dispersion in scaled entropy in the inner regions, 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 constant 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 unimodal with a median value of 0.98, and there is a clear correlation of outer slope with temperature. Renormalising the dimensionless entropy profiles by the gas mass fraction profile fgas (<R), leads to a remarkable reduction in the scatter, implying that 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 centrally 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. The observed normalisation, however, would suggest entropy modification 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.
ABSTRACT
Entropy is an advantageous diagnostics to study the thermodynamic history of the intracluster plasma of galaxy clusters. We present the entropy profile of the Abell 2244 galaxy cluster ...derived both exclusively using X-ray data from the low-background Swift XRT telescope and using the Planck y data. The entropy profile derivation using X-rays only is robust at least to the virial radius because the cluster brightness is large compared to the X-ray background at low energies, the temperature is strongly bounded by the lack of cluster X-ray photons at energies kT > 3 keV, and the XRT background is low, stable, and understood. In the observed solid angle, about one quadrant, the entropy radial profile deviates from a power law at the virial radius, mainly because of a sharp drop in the cluster temperature. This bending of the entropy profile is confirmed when X-ray spectral information is replaced by the Compton map. Clumping and non-thermal pressure support are insufficient to restore a power-law entropy profile because they are bound to be small by: (i) the agreement between mass estimates from different tracers (gas and galaxies), (ii) the agreement between entropy profile determinations based on combinations of observables with different sensitivities and systematics, and (iii) the low value of clumping as estimated using the azimuthal scatter and the gas fraction. Based on numerical simulations, ion–electron equilibration is also insufficient to restore a linear entropy profile. Therefore, the bending of the entropy profiles seems to be robustly derived and witnesses the theoretically predicted decrease in the inflow through the virial boundary.
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