From the core to the outskirts: structure analysis of three massive galaxy clusters

The hierarchical model of structure formation is a key prediction of the Lambda 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 RXCJ0225.9-4154, RXCJ0528.9-3927, and RXCJ2308.3-0211, as part of a sample of massive X-ray luminous clusters located at intermediate redshifts. 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 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. RXCJ0225 has a bimodal 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. RXCJ0528 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. RXCJ2308 has complex central dynamics, and it is found at the core of a superstes-cluster. The complexity of the cluster's central dynamics reflects the richness of its large-scale environment: RXCJ0225 and RXCJ2308 present a mass fraction in substructures larger than the typical 0.05-0.15, whereas the isolated cluster RXCJ0528 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

Probing the evolution of the substructure frequency in galaxy clusters up to z~1

Context. Galaxy clusters are the last and largest objects to form in the standard hierarchical structure formation scenario through merging of smaller systems. The substructure frequency in the past and present epoch provides excellent means for studying the underlying cosmological model. Aims. Using X-ray observations, we study the substructure frequency as a function of redshift by quantifying and comparing the fraction of dynamically young clusters at different redshifts up to z=1.08. We are especially interested in possible biases due to the inconsistent data quality of the low-z and high-z samples. Methods. Two well-studied morphology estimators, power ratio P3/P0 and center shift w, were used to quantify the dynamical state of 129 galaxy clusters, taking into account the different observational depth and noise levels of the observations. Results. Owing to the sensitivity of P3/P0 to Poisson noise, it is essential to use datasets with similar photon statistics when studying the P3/P0-z relation. We degraded the high-quality data of the low-redshift sample to the low data quality of the high-z observations and found a shallow positive slope that is, however, not significant, indicating a slightly larger fraction of dynamically young objects at higher redshift. The w-z relation shows no significant dependence on the data quality and gives a similar result. Conclusions. We find a similar trend for P3/P0 and w, namely a very mild increase of the disturbed cluster fraction with increasing redshifts. Within the significance limits, our findings are also consistent with no evolution.Comment: A&A in pres

The extended ROSAT-ESO Flux Limited X-ray Galaxy Cluster Survey (REFLEX II) IV. X-ray Luminosity Function and First Constraints on Cosmological Parameters

The X-ray luminosity function is an important statistic of the census of galaxy clusters and an important means to probe the cosmological model of our Universe. Based on our recently completed REFLEX II cluster sample we construct the X-ray luminosity function of galaxy clusters for several redshift slices from $z = 0$ to $z = 0.4$ and discuss its implications. We find no significant signature of redshift evolution of the luminosity function in the redshift interval. We provide the results of fits of a parameterized Schechter function and extensions of it which provide a reasonable characterization of the data. Using a model for structure formation and galaxy cluster evolution we compare the observed X-ray luminosity function with predictions for different cosmological models. For the most interesting constraints for the cosmological parameters $\Omega_m$ and $\sigma_8$ we obatain $\Omega_m \sim 0.27 \pm 0.03$ and $\sigma_8 \sim 0.80 \pm 0.03$ based on the statistical uncertainty alone. Marginalizing over the most important uncertainties, the normalisation and slope of the $L_X - M$ scaling relation, we find $\Omega_m \sim 0.29 \pm 0.04$ and $\sigma_8 \sim 0.77 \pm 0.07$ ($1\sigma$ confidence limits). We compare our results with those of the SZ-cluster survey provided by the PLANCK mission and we find very good agreement with the results using PLANCK clusters as cosmological probes, but we have some tension with PLANCK cosmological results from the microwave background anisotropies. We also make a comparison with other cluster surveys. We find good agreement with these previous results and show that the REFLEX II survey provides a significant reduction in the uncertainties compared to earlier measurements.Comment: Submitted for publication to Astronomy and Astrophysics, 15 pages, 17 figure

Optical and X-ray profiles in the REXCESS sample of galaxy clusters

Galaxy clusters' structure, dominated by dark matter, is traced by member galaxies in the optical and hot intra-cluster medium (ICM) in X-rays. We compare the radial distribution of these components and determine the mass-to-light ratio vs. system mass relation. We use 14 clusters from the REXCESS sample which is representative of clusters detected in X-ray surveys. Photometric observations with the Wide Field Imager on the 2.2m MPG/ESO telescope are used to determine the number density profiles of the galaxy distribution out to $r_{200}$. These are compared to electron density profiles of the ICM obtained using XMM-Newton, and dark matter profiles inferred from scaling relations and an NFW model. While red sequence galaxies trace the total matter profile, the blue galaxy distribution is much shallower. We see a deficit of faint galaxies in the central regions of massive and regular clusters, and strong suppression of bright and faint blue galaxies in the centres of cool-core clusters, attributable to ram pressure stripping of gas from blue galaxies in high density regions of ICM and disruption of faint galaxies due to galaxy interactions. We find a mass-to-light ratio vs. mass relation within $r_{200}$ of $\left(3.0\pm0.4\right) \times 10^2\, h\,\mathrm{M}_{\odot}\,\mathrm{L}_{\odot}^{-1}$ at $10^{15}\,\mathrm{M}_{\odot}$ with slope $0.16 \pm 0.14$, consistent with most previous results

Suzaku observations of the low surface brightness cluster A76

Context: We present results of Suzaku observations of a nearby galaxy cluster A76 at z=0.0395. This cluster is characterized by extremely low X-ray surface brightness and is hereafter referred to as the LSB cluster. Aims: To understand the nature and thermodynamic evolution of the LSB cluster by studying the physical properties of the hot intracluster medium in A76. Methods: We conducted two-pointed Suzaku observations of A76 and examined the global gas properties of the cluster by XIS spectral analysis. We also performed deprojection analysis of annular spectra and derived radial profiles of gas temperature, density and entropy out to approximately 850 kpc (~ 0.6 r_200) and 560 kpc (~0.4 r_200) in A76 East and A76 West, respectively. Results: The measured global temperature and metal abundance are approximately 3.3 keV and 0.24 solar, respectively. From the deprojection analysis, the entropy profile is found to be flat with respect to radius. The entropy within the central region (r < 0.2r_200) is exceptionally high (~400 keV cm^2). This phenomenon is not readily explained by either gravitational heating or preheating. The X-ray morphology is clumped and irregular, and the electron density is extremely low (1e-4 -- 1e-3 cm^-3) for the observed high temperature, suggesting that A76 is in the early phase of cluster formation and the gas compression due to gravitational potential confinement is lagging behind the gas heating.Comment: 7 pages, 5 figures, A&A accepte

Error analysis of quadratic power spectrum estimates for CMB polarization: sampling covariance

Quadratic methods with heuristic weighting (e.g. pseudo-C_l or correlation function methods) represent an efficient way to estimate power spectra of the cosmic microwave background (CMB) anisotropies and their polarization. We construct the sample covariance properties of such estimators for CMB polarization, and develop semi-analytic techniques to approximate the pseudo-C_l sample covariance matrices at high Legendre multipoles, taking account of the geometric effects of mode coupling and the mixing between the electric (E) and magnetic (B) polarization that arise for observations covering only part of the sky. The E-B mixing ultimately limits the applicability of heuristically-weighted quadratic methods to searches for the gravitational-wave signal in the large-angle B-mode polarization, even for methods that can recover (exactly) unbiased estimates of the B-mode power. We show that for surveys covering one or two per cent of the sky, the contribution of E-mode power to the covariance of the recovered B-mode power spectrum typically limits the tensor-to-scalar ratio that can be probed with such methods to around 0.05.Comment: 26 pages, 10 figures (some reduced-resolution postscript; better version of Fig. 4 also included with submission but not embedded). Minor changes (and typos fixed) to match published (MNRAS) versio

The dynamical state of RXCJ1230.7+3439: a multi-substructured merging galaxy cluster

We analyse the kinematical and dynamical state of the galaxy cluster RXCJ1230.7+3439, at z=0.332, using 93 new spectroscopic redshifts of galaxies acquired at the 3.6m TNG telescope and from SDSS DR16 public data. We find that RXCJ1230 appears as a clearly isolated peak in the redshift space, with a global line-of-sight velocity dispersion of $1004_{-122}^{+147}$ km s$^{-1}$, and showing a very complex structure with the presence of three subclusters. Our analyses confirm that the three substructures detected are in a pre-merger phase, where the main interaction takes place with the south-west subclump. We compute a velocity dispersion of $\sigma_\textrm{v} \sim 1000$ and $\sigma_\textrm{v} \sim 800$ km s$^{-1}$ for the main cluster and the south-west substructure, respectively. The central main body and south-west substructure differ by $\sim 870$ km s$^{-1}$ in the LOS velocity. From these data, we estimate a dynamical mass of $M_{200}= 9.0 \pm 1.5 \times 10^{14}$ M$_{\odot}$ and $4.4 \pm 3.3 \times 10^{14}$ M$_{\odot}$ for the RXCJ1230 main body and south-west clump, respectively, which reveals that the cluster will suffer a merging characterized by a 2:1 mass ratio impact. We solve a two-body problem for this interaction and find that the most likely solution suggests that the merging axis lies almost contained in the plane of the sky and the subcluster will fully interact in $\sim0.3$ Gyr. The comparison between the dynamical masses and those derived from X-ray data reveals a good agreement within errors (differences $\sim 15$\%), which suggests that the innermost regions ($<r_{500}$) of the galaxy clumps are almost in hydrostatical equilibrium. To summarize, RXCJ1230 is a young but also massive cluster in a pre-merging phase accreeting other galaxy systems from its environment.Comment: To be published in A&

Observational evidence for a local underdensity in the Universe and its effect on the measurement of the Hubble constant

For precision cosmological studies it is important to know the local properties of the reference point from which we observe the Universe. Particularly for the determination of the Hubble constant with low-redshift distance indicators, the values observed depend on the average matter density within the distance range covered. In this study we used the spatial distribution of galaxy clusters to map the matter density distribution in the local Universe. The study is based on our CLASSIX galaxy cluster survey, which is highly complete and well characterised, where galaxy clusters are detected by their X-ray emission. In total, 1653 galaxy clusters outside the "zone of avoidance"fulfil the selection criteria and are involved in this study. We find a local underdensity in the cluster distribution of about 30-60% which extends about 85 Mpc to the north and ∼170 Mpc to the south. We study the density distribution as a function of redshift in detail in several regions in the sky. For three regions for which the galaxy density distribution has previously been studied, we find good agreement between the density distribution of clusters and galaxies. Correcting for the bias in the cluster distribution we infer an underdensity in the matter distribution of about -30 ± 15% (-20 ± 10%) in a region with a radius of about 100 (∼140) Mpc. Calculating the probability of finding such an underdensity through structure formation theory in a ΛCDM universe with concordance cosmological parameters, we find a probability characterised by σ-values of 1.3 - 3.7. This indicates low probabilities, but with values of around 10% at the lower uncertainty limit, the existence of an underdensity cannot be ruled out. Inside this underdensity, the observed Hubble parameter will be larger by about 5.5 +2.1-2.8%, which explains part of the discrepancy between the locally measured value of H0 compared to the value of the Hubble parameter inferred from the Planck observations of cosmic microwave background anisotropies. If distance indicators outside the local underdensity are included, as in many modern analyses, this effect is diluted. © H. Böhringer et al. 2019

The effect of AGN feedback on the X-ray morphologies of clusters: Simulations vs. observations

Clusters of galaxies probe the large-scale distribution of matter and are a useful tool to test the cosmological models by constraining cosmic structure growth and the expansion of the Universe. It is the scaling relations between mass observables and the true mass of a cluster through which we obtain the cosmological constraints by comparing to theoretical cluster mass functions. These scaling relations are, however, heavily influenced by cluster morphology. The presence of the slight tension in recent cosmological constraints on $\Omega_m$ and $\sigma_8$ based on the CMB and clusters has boosted the interests in looking for possible sources for the discrepancy. Therefore we study here the effect of active galactic nucleus (AGN) feedback as one of the major mechanisms modifying the cluster morphology influencing scaling relations. It is known that AGN feedback injects energies up to 10$^{62}$ erg into the intracluster medium, controls the heating and cooling of a cluster, and re-distributes cold gas from the centre to outer radii. We have also learned that cluster simulations with AGN feedback can reproduce observed cluster properties, for example, the X-ray luminosity, temperature, and cooling rate at the centre better than without the AGN feedback. In this paper using cosmological hydrodynamical simulations we investigate how the AGN feedback changes the X-ray morphology of the simulated systems, and compare this to the observed Representative $\textit{XMM-Newton}$ Cluster Structure Survey (REXCESS) clusters. We apply two substructure measures, centre shifts ($\omega$) and power ratios (e.g. $P_3$/$P_0$), to characterise the cluster morphology, and find that our simulated clusters are more substructured than the observed clusters based on the values of $\omega$ and $P_3$/$P_0$. We also show that the degree of this discrepancy is affected by the inclusion of AGN feedback. While the clusters simulated with the AGN feedback are in much better agreement with the REXCESS $L_X−T$ relation, they are also more substructured, which increases the tension with observations. When classified as non-relaxed or relaxed according to their $\omega$ and $P_3$/$P_0$ values, we find that there are no relaxed clusters in the simulations with the AGN feedback. This suggests that not only global cluster properties, like $L_X$ and T, and radial profiles should be used to compare and to calibrate simulations with observations, but also substructure measures like centre shifts and power ratios. Finally, we discuss what changes in the simulations might ease the tension with observational constraints on these quantities.H.B. and G.C. acknowledge support from the DFG Transregio Program TR33 and the Munich Excellence Cluster “Structure and Evolution of the Universe”. G.C. acknowledges support by the DLR under grant No. 50 OR 1405. E.P. acknowledges support by the Kavli Foundation and the FP7 ERC Advanced Grant Emergence-320596

The extended ROSAT-ESO flux-limited X-ray galaxy cluster survey (REFLEX II): V. Exploring a local underdensity in the southern sky

Several claims have been made that we are located in a locally underdense region of the Universe based on observations of supernovae and galaxy density distributions. Two recent studies of K-band galaxy surveys have, in particular, provided new support for a local underdensity in the galaxy distribution out to distances of 200-300 Mpc. If confirmed, such local underdensities would have important implications interpreting local measurements of cosmological parameters. Galaxy clusters have been shown to be ideal probes for tracing the large-scale structure of the Universe. In this paper we study the local density distribution in the southern sky with the X-ray detected galaxy clusters from the REFLEX II cluster survey. From the normalised comoving number density of clusters, we find an average underdensity of ∼30-40% in the redshift range out to z ∼ 0.04 (∼170 Mpc) in the southern extragalactic sky with a significance greater than 3.4σ. On larger scales from 300 Mpc to over 1 Gpc, the density distribution appears remarkably homogeneous. The local underdensity seems to be dominated by the south Galactic cap region. A comparison of the cluster distribution with that of galaxies in the K -band from a recent study shows that galaxies and clusters trace each other very closely in density. In the south Galactic cap region both surveys find a local underdensity in the redshift range z = 0 to 0.05 and no significant underdensity in the north Galactic cap at southern latitudes. Cosmological models that attempt to interpret the cosmic acceleration, deduced from observations of type Ia supernovae, by a large local void without the need for reacceleration, require that we are located close to the centre of a roughly spherical void with a minimum size of ∼300 Mpc. In contrast our results show that the local underdensity is not isotropic and limited to a size significantly smaller than 300 Mpc radius