Context. The hot plasma in a galaxy cluster is expected to be heated to high temperatures through shocks and adiabatic compression. The thermodynamical properties of the gas encode information on the ...processes leading to the thermalization of the gas in the cluster’s potential well and on non-gravitational processes such as gas cooling, AGN feedback, shocks, turbulence, bulk motions, cosmic rays and magnetic field. Aims. In this work we present the radial profiles of the thermodynamic properties of the intracluster medium (ICM) out to the virial radius for a sample of 12 galaxy clusters selected from the Planck all-sky survey. We determine the universal profiles of gas density, temperature, pressure, and entropy over more than two decades in radius, from 0.01R500 to 2R500. Methods. We exploited X-ray information from XMM-Newton and Sunyaev-Zel’dovich constraints from Planck to recover thermodynamic properties out to 2R500. We provide average functional forms for the radial dependence of the main quantities and quantify the slope and intrinsic scatter of the population as a function of radius. Results. We find that gas density and pressure profiles steepen steadily with radius, in excellent agreement with previous observational results. Entropy profiles beyond R500 closely follow the predictions for the gravitational collapse of structures. The scatter in all thermodynamical quantities reaches a minimum in the range 0.2 − 0.8R500 and increases outward. Somewhat surprisingly, we find that pressure is substantially more scattered than temperature and density. Conclusions. Our results indicate that once accreting substructures are properly excised, the properties of the ICM beyond the cooling region (R > 0.3R500) follow remarkably well the predictions of simple gravitational collapse and require few non-gravitational corrections.
Galaxy clusters are the endpoints of structure formation and are continuously growing through the merging and accretion of smaller structures. Numerical simulations predict that a fraction of their ...energy content is not yet thermalized, mainly in the form of kinetic motions (turbulence, bulk motions). Measuring the level of non-thermal pressure support is necessary to understand the processes leading to the virialization of the gas within the potential well of the main halo and to calibrate the biases in hydrostatic mass estimates. We present high-quality measurements of hydrostatic masses and intracluster gas fraction out to the virial radius for a sample of 13 nearby clusters with available XMM-Newton and Planck data. We compare our hydrostatic gas fractions with the expected universal gas fraction to constrain the level of non-thermal pressure support. We find that hydrostatic masses require little correction and infer a median non-thermal pressure fraction of ∼6% and ∼10% at R500 and R200, respectively. Our values are lower than the expectations of hydrodynamical simulations, possibly implying a faster thermalization of the gas. If instead we use the mass calibration adopted by the Planck team, we find that the gas fraction of massive local systems implies a mass bias 1 − b = 0.85 ± 0.05 for Sunyaev–Zeldovich-derived masses, with some evidence for a mass-dependent bias. Conversely, the high bias required to match Planck cosmic microwave background and cluster count cosmology is excluded by the data at high significance, unless the most massive halos are missing a substantial fraction of their baryons.
Abstract
We characterized the population of galaxy clusters detected with the Sunyaev–Zeldovich (SZ) effect with Planck by measuring the cool-core state of the objects in a well-defined subsample of ...the Planck SZ catalogue. We used as an indicator the concentration parameter. The fraction of cool-core clusters is 29 ± 4 per cent and does not show significant indications of evolution in the redshift range covered by our sample. We compare the distribution of the concentration parameter in the Planck sample with the one of the X-ray selected sample MACS: the distributions are significantly different and the cool-core fraction in MACS is much higher (59 ± 5 per cent) than that in Planck. Since X-ray-selected samples are known to be biased towards cool cores due to the presence of their prominent surface brightness peak, we simulated the impact of the ‘cool-core bias’. We found that this bias plays a large role in the difference between the fractions of cool cores in the two samples. We examined other selection effects that could in principle bias SZ surveys against cool cores, but we found that their impact is not sufficient to explain the difference between Planck and MACS. The population of X-ray underluminous objects, which are found in SZ surveys but missing in X-ray samples, could possibly contribute to the difference, as we found most of them to be non-cool cores, but this hypothesis deserves further investigation.
Aims. We measure radial temperature profiles as far out as possible for a sample of ≈50 hot, intermediate redshift galaxy clusters, selected from the XMM-Newton archive, keeping systematic errors ...under control. Methods. Our work is characterized by two major improvements. First, we used background modeling, rather than background subtraction, and the Cash statistic rather than the $\chi^2$. This method requires a careful characterization of all background components. Second, we assessed systematic effects in detail. We performed two groups of tests. Prior to the analysis, we made use of extensive simulations to quantify the impact of different spectral components on simulated spectra. After the analysis, we investigated how the measured temperature profile changes, when choosing different key parameters. Results. The mean temperature profile declines beyond 0.2 R180. For the first time, we provide an assessment of the source and the magnitude of systematic uncertainties. When comparing our profile with those obtained from hydrodynamic simulations, we find the slopes beyond ≈0.2 R180 to be similar. Our mean profile is similar but somewhat flatter with respect to those obtained by previous observational works, possibly as a consequence of a different level of characterizing systematic effects. Conclusions. This work allows us to not only constrain cluster temperature profiles in outer regions with confidence, but also, from a more general point of view, to explore the limits of the current X-ray experiments (in particular XMM-Newton) with respect to the analysis of low surface-brightness emission.
Aims. We measured radial metallicity profiles for a sample of ≈50 hot, intermediate redshift galaxy clusters, selected from the XMM-Newton archive. Methods. As in our previous paper, we used ...background modeling rather than background subtraction, and the Cash statistic rather than the $\chi^2$. This method requires a careful characterization of all background components. We also performed Montecarlo simulations to assess systematic effects. Results. The mean metallicity profile shows a peak in the center, and gently declines out to 0.2 R180. Beyond 0.2 R180 the metallicity is ≈0.2 solar and, at variance with recently published expectations based on simulations, consistent with being flat. We find no evidence of profile evolution from $z = 0.1$ to $z = 0.3$. When comparing our mean profile to those obtained by recent works with BeppoSAX and Chandra, we find remarkable agreement over the entire radial range.
We have measured the spectrum of the Cosmic X-ray Background (CXB) in the 2-8 keV range with the high throughput EPIC/MOS instrument onboard XMM-Newton. A large sample of high galactic latitude ...observations was used, covering a total solid angle of 5.5 square degrees. Our study is based on a very careful characterization and subtraction of the instrumental background, which is crucial for a robust measurement of the faintest diffuse source of the X-ray sky. The CXB spectrum is consistent with a power law having a photon index Gamma = 1.41 plus or minus 0.06 and a normalization of 2.46 plus or minus 0.09 photons cm super(-2) s super(-1) sr super(-1) keV super(-1) at 3 keV ( similar to 11.6 photons cm super(-2) s super(-1) sr super(-1) keV super(-1) at 1 keV), corresponding to a 2-10 keV flux of (2.24 plus or minus 0.16) x 10 super(-11) erg cm super(-2) s super(-1) deg super(-1) (90% confidence level, including the absolute flux calibration uncertainty). Our results are in excellent agreement with two of the most recent CXB measurements, performed with BeppoSAX LECS/MECS data (Vecchi et al. 1999) and with an independent analysis of XMM-Newton EPIC/MOS data (Lumb et al. 2002), providing a very strong constraint to the absolute sky surface brightness in this energy range, so far affected by an similar to 40% uncertainty. Our measurement implies that the fraction of CXB resolved by the recent deep X-ray observations in the 2-10 keV band is of 80 plus or minus 7% (1 sigma ), suggesting the existence of a new population of faint sources, largely undetected within the current sensitivity limits of the deepest X-ray surveys.
Context. Galaxy clusters are continuously growing through the accretion of matter in their outskirts. This process induces inhomogeneities in the gas density distribution (clumping) that need to be ...taken into account to recover the physical properties of the intracluster medium (ICM) at large radii. Aims. We studied the thermodynamic properties in the outskirts (R > R500) of the massive galaxy cluster Abell 2142 by combining the Sunyaev Zel’dovich (SZ) effect with the X-ray signal. Methods. We combined the SZ pressure profile measured by Planck with the XMM-Newton gas density profile to recover radial profiles of temperature, entropy, and hydrostatic mass out to 2 × R500. We used a method that is insensitive to clumping to recover the gas density, and we compared the results with traditional X-ray measurement techniques. Results. When taking clumping into account, our joint X-SZ entropy profile is consistent with the predictions from pure gravitational collapse, whereas a significant entropy flattening is found when the effect of clumping is neglected. The hydrostatic mass profile recovered using joint X-SZ data agrees with that obtained from spectroscopic X-ray measurements and with mass reconstructions obtained through weak lensing and galaxy kinematics. Conclusions. We found that clumping can explain the entropy flattening observed by Suzaku in the outskirts of several clusters. When using a method that is insensitive to clumping for the reconstruction of the gas density, the thermodynamic properties of Abell 2142 are compatible with the assumption that the thermal gas pressure sustains gravity and that the entropy is injected at accretion shocks, with no need to evoke more exotic physics. Our results highlight the need for X-ray observations with sufficient spatial resolution, and large collecting area, to understand the processes at work in cluster outer regions.
We want to characterize the dynamical state of galaxy clusters detected with the Sunyaev–Zeldovich (SZ) effect by Planck and compare them with the dynamical state of clusters selected in X-rays ...survey. We analysed a representative subsample of the Planck SZ catalogue, containing the 132 clusters with the highest signal to noise ratio and characterize their dynamical state using as an indicator the projected offset between the peak of the X-ray emission and the position of the Brightest cluster galaxy. We compare the distribution of this indicator for the Planck SZ-selected sample and three X-ray-selected samples (HIFLUGCS, MACS and REXCESS). The distributions are significantly different and the fraction of relaxed objects is smaller in the Planck sample (52 ± 4 per cent) than in X-ray samples (≃74 per cent) We interpret this result as an indication of different selection effects affecting X-rays (e.g. ‘cool core bias’) and SZ surveys of galaxy clusters.
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