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.
Aims. Imaging and spectroscopy of X-ray extended sources require a proper characterisation of a spatially unresolved background signal. This background includes sky and instrumental components, each ...of which are characterised by its proper spatial and spectral behaviour. While the X-ray sky background has been extensively studied in previous work, here we analyse and model the instrumental background of the ACIS-I detector on board the Chandra X-ray observatory in very faint mode. Methods. Caused by interaction of highly energetic particles with the detector, the ACIS-I instrumental background is spectrally characterised by the superimposition of several fluorescence emission lines onto a continuum. To isolate its flux from any sky component, we fitted an analytical model of the continuum to observations performed in very faint mode with the detector in the stowed position shielded from the sky, and gathered over the eight-year period starting in 2001. The remaining emission lines were fitted to blank-sky observations of the same period. We found 11 emission lines. Analysing the spatial variation of the amplitude, energy and width of these lines has further allowed us to infer that three lines of these are presumably due to an energy correction artefact produced in the frame store. Results. We provide an analytical model that predicts the instrumental background with a precision of 2% in the continuum and 5% in the lines. We use this model to measure the flux of the unresolved cosmic X-ray background in the Chandra deep field south. We obtain a flux of 10.2+0.5-0.4 × 10-13 erg cm-2 deg-2 s-1 for the 1−2 keV band and (3.8 ± 0.2) × 10-12 erg cm-2 deg-2 s-1 for the 2−8 keV band.
A521 is an interacting galaxy cluster located at z = 0.247, hosting a low-frequency radio halo connected to an eastern radio relic. Previous Chandra observations hinted at the presence of an X-ray ...brightness edge at the position of the relic, which may be a shock front. We analyze a deep observation of A521 recently performed with XMM-Newton in order to probe the cluster structure up to the outermost regions covered by the radio emission. The cluster atmosphere exhibits various brightness and temperature anisotropies. In particular, two cluster cores appear to be separated by two cold fronts. We find two shock fronts, one that was suggested by Chandra and that is propagating to the east, and another to the southwestern cluster outskirt. The two main interacting clusters appear to be separated by a shock-heated region, which exhibits a spatial correlation with the radio halo. The outer edge of the radio relic coincides spatially with a shock front, suggesting that this shock is responsible for the generation of cosmic-ray electrons in the relic. The propagation direction and Mach number of the shock front derived from the gas density jump, M = 2.4 + or - 0.2, are consistent with expectations from the radio spectral index, under the assumption of Fermi I acceleration mechanism.
Abstract
We present the first detailed analysis of the ultra-steep spectrum radio halo in the merging galaxy cluster A521, based on upgraded Giant Metrewave Radio telescope observations. The ...combination of radio observations (300–850 MHz) and archival X-ray data provides a new window into the complex physics occurring in this system. When compared to all previous analyses, our sensitive radio images detected the centrally located radio halo emission to a greater extent of ∼1.3 Mpc. A faint extension of the southeastern radio relic has been discovered. We detected another relic, recently discovered by MeerKAT, and coincident with a possible shock front in the X-rays, at the northwest position of the center. We find that the integrated spectrum of the radio halo is well-fitted with a spectral index of −1.86 ± 0.12. A spatially resolved spectral index map revealed the spectral index fluctuations, as well as an outward radial steepening of the average spectral index. The radio and X-ray surface brightness is well correlated for the entire and different subparts of the halo, with sublinear correlation slopes (0.50–0.65). We also found a mild anticorrelation between the spectral index and X-ray surface brightness. Newly detected extensions of the SE relic and the counter relic are consistent with the merger in the plane of the sky.
Abstract
We present LOw Frequency ARray observations of the Coma Cluster field at 144 MHz. The cluster hosts one of the most famous radio halos, a relic, and a low surface brightness bridge. We ...detect new features that allow us to make a step forward in the understanding of particle acceleration in clusters. The radio halo extends for more than 2 Mpc, which is the largest extent ever reported. To the northeast of the cluster, beyond the Coma virial radius, we discover an arc-like radio source that could trace particles accelerated by an accretion shock. To the west of the halo, coincident with a shock detected in the X-rays, we confirm the presence of a radio front, with different spectral properties with respect to the rest of the halo. We detect a radial steepening of the radio halo spectral index between 144 and 342 MHz, at ∼30′ from the cluster center, that may indicate a non-constant re-acceleration time throughout the volume. We also detect a mild steepening of the spectral index toward the cluster center. For the first time, a radial change in the slope of the radio–X-ray correlation is found, and we show that such a change could indicate an increasing fraction of cosmic-ray versus thermal energy density in the cluster outskirts. Finally, we investigate the origin of the emission between the relic and the source NGC 4789, and we argue that NGC 4789 could have crossed the shock originating the radio emission visible between its tail and the relic.
Successive releases of Planck data have demonstrated the strength of the Sunyaev-Zeldovich (SZ) effect in detecting hot baryons out to the galaxy cluster peripheries. To infer the hot gas pressure ...structure from nearby galaxy clusters to more distant objects, we developed a parametric method that models the spectral energy distribution and spatial anisotropies of both the Galactic thermal dust (GTD) and the cosmic microwave background (CMB), which are combined with the cluster SZ and dust signals. Taking advantage of the best angular resolution of the High Frequency Instrument channels (5 arcmin) and using X-ray priors in the innermost cluster regions that are not resolved with Planck, this modeling allowed us to analyze a sample of 61 nearby members of the Planck Catalogue of SZ sources ( , ) using the full mission data, as well as to examine a distant sample of 23 clusters ( , ) that have been recently followed-up with XMM-Newton and Chandra observations. We find that (i) the average shape of the mass-scaled pressure profiles agrees with results obtained by the Planck Collaboration in the nearby cluster sample, and that (ii) no sign of evolution is discernible between averaged pressure profiles of the low- and high-redshift cluster samples. In line with theoretical predictions for these halo masses and redshift ranges, the dispersion of individual profiles relative to a self-similar shape stays well below 10% inside r500 but increases in the cluster outskirts.
We test the assumption of hydrostatic equilibrium in an X-ray luminosity selected sample of 50 galaxy clusters at 0.15 < z < 0.3 from the Local Cluster Substructure Survey (LoCuSS). Our weak-lensing ...measurements of M
500 control systematic biases to sub-4 per cent, and our hydrostatic measurements of the same achieve excellent agreement between XMM–Newton and Chandra. The mean ratio of X-ray to lensing mass for these 50 clusters is
$\mathrel {\beta _{\rm X}}\;= 0.95\pm 0.05$
, and for the 44 clusters also detected by Planck, the mean ratio of Planck mass estimate to LoCuSS lensing mass is
$\mathrel {\beta _{\rm P}}\;= 0.95\pm 0.04$
. Based on a careful like-for-like analysis, we find that LoCuSS, the Canadian Cluster Comparison Project, and Weighing the Giants agree on
$\mathrel {\beta _{\rm P}}\;\simeq 0.9{\rm -}0.95$
at 0.15 < z < 0.3. This small level of hydrostatic bias disagrees at ∼5σ with the level required to reconcile Planck cosmology results from the cosmic microwave background and galaxy cluster counts.
Aims.Using a newly developed algorithm, we map, to the highest angular resolution allowed by the data, the temperature structure of the intra-cluster medium (ICM) within a nearly complete X-ray flux ...limited sample of galaxy clusters in the redshift range between ${z}=0.045$ and ${z}=0.096$. Our sample contains seven bright clusters of galaxies observed with XMM-Newton: Abell 399, Abell 401, Abell 478, Abell 1795, Abell 2029, Abell 2065, Abell 2256. Methods.We use a multi-scale spectral mapping algorithm especially designed to map spectroscopic observables from X-ray extended emission of the ICM. By means of a wavelet analysis, this algorithm couples spatially resolved spectroscopy with a structure detection approach. Derived from a former algorithm using Haar wavelets, our algorithm is now implemented with B-spline wavelets in order to perform a more regular analysis of the signal. Compared to other adaptive algorithms, our method has the advantage of analysing spatially the gas temperature structure itself, instead of being primarily driven by the geometry of gas brightness. Results.For the four clusters in our sample that are major mergers, we find a rather complex thermal structure with strong thermal variations consistent with their dynamics. For two of them, A2065 and A2256, we perform a 3-d analysis of cold front-like features evidenced from the gas temperature and brightness maps. Furthermore, we detect a significant non-radial thermal structure outside the cool core region of the other 3 more “regular” clusters, with relative amplitudes of about about 10% and typical sizes ranging between 2 and 3 arcmin. We investigate possible implications of this thermal structure on the mass estimates, by extracting the surface brightness and temperature profiles from complementary sectors in the “regular” clusters A1795 and A2029, corresponding to hottest and coldest regions in the maps. For A2029, the temperature and surface brightness gradients seem to compensate each other, leading to a consistent mass profile. For A1795, however, the temperature structure leads to a significant mass discrepancy in the innermost cluster region. The third “regular” cluster, A478, is located in a particular sky region characterised by strong variations of neutral hydrogen column density, Nh, even on angular scales smaller than the cluster itself. For this cluster, we derive a spectroscopic Nh map and investigate the origin of Nh structure by discussing its correlation with galactic emission of dust in the infrared.
Using a MeerKAT observation of the galaxy cluster A3562 (a member of the Shapley Supercluster), we have discovered a narrow, long and straight, very faint radio filament, which branches out at a ...straight angle from the tail of a radio galaxy located in projection near the core of the cluster. The radio filament spans 200 kpc and aligns with a sloshing cold front seen in the X-rays, staying inside the front in projection. The radio spectral index along the filament appears uniform (within large uncertainties) atα' −1.5. We propose that the radio galaxy is located outside the cold front, but dips its tail under the front. The tangential wind that blows there may stretch the radio plasma from the radio galaxy into a filamentary structure. Some reacceleration is needed in this scenario to maintain the radio spectrum uniform. Alternatively, the cosmic ray electrons from that spot in the tail can spread along the cluster magnetic field lines, straightened by that same tangential flow, via anomalously fast diffusion. Our radio filament can provide constraints on this process. We also uncover a compact radio source at the Brightest Cluster Galaxy (BCG) that is 2–3 orders of magnitude
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