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 a detailed kinematical and dynamical study of the galaxy cluster RXCJ1111.6+4050 (RXCJ1111), at
z
= 0.0756 using 104 new spectroscopic redshifts of galaxies observed at the Telescopio ...Nazionale
Galileo
and SDSS DR16 public archive. Our analysis is performed in a multiwavelength context in order to study and compare mainly optical and X-ray properties using
XMM-Newton
data. We find that RXCJ1111 is a galaxy cluster showing a velocity distribution with clear deviations from Gaussianity, that we are able to explain by the presence of a substructure within the cluster. The two cluster components show velocity dispersions of 644 ± 56 km s
−1
and 410 ± 123 km s
−1
, which yield dynamical masses of
M
200
= 1.9 ± 0.4 × 10
14
M
⊙
and 0.6 ± 0.4 × 10
14
M
⊙
for the main system and substructure, respectively. The 2D spatial distribution of galaxies and X-ray surface brightness of RXCJ1111 presents an elongation in the North–South direction. These observational facts, together with a gradient of 250−350 km s
−1
Mpc
−1
in the velocity field, following the NNE–SSE direction, suggest that the merger axis between the main system and substructure is slightly tilted with respect to the line-of-sight. The substructure is characterized by a magnitude gap Δ
m
12
≥ 1.8, so it fits the “fossil-like” definition of a galaxy group. From the X-ray observations, we estimate a
M
500, X
= 1.68 ± 0.25 × 10
14
M
⊙
, which is in good agreement with the dynamical masses when two galaxy components are considered separately. This suggests that the mass estimates obtained from X-ray and velocity dispersion are compatible even for non-relaxed clusters, at least when we are able to identify and separate galaxy clumps and derive masses by considering the virialized regions. We propose a 3D merging model and find that the fossil group is in an early phase of collision with the RXCJ1111 main cluster and placed at ∼8° ( ± 3° ) from line-of-sight. This merging model would explain the slight increase found in the
T
X
with respect to what we would expect for relaxed clusters. Due to the presence of several brightest galaxies, after this collision, the substructure would presumably lose its fossil condition. Therefore, RXCJ1111 represents the observational evidence that the fossil stage of a system can be temporary and transitional.
ABSTRACT
During cluster mergers, the intracluster gas and member galaxies undergo dynamic evolution, but at different time-scales and reach different states. We collect 24 galaxy clusters in ...quasi-equilibrium state as indicated by the X-ray image, and calculate the cluster orientations and three kinds of dynamical parameters, i.e. the normalized centroid offset, the sphere index, and the ellipticity, for these clusters from the distributions of member galaxies and also the intracluster gas. We find consistent alignments for the orientations estimated from the two components. However, the three kinds of dynamical parameters indicated by member galaxies are systematically larger than those derived from the gas component, suggesting that the gas component is more relaxed than member galaxies. Differences of dynamical features between the intracluster gas and member galaxies are independent of cluster mass and concentration. We conclude that the intracluster gas reaches the dynamic equilibrium state earlier than the almost-collisionless member galaxies.
Context. The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure and the related scaling ...relations between observables and cluster mass. A large effort has therefore been made to compile global galaxy cluster properties, in particular those obtained through X-ray observations, and to study their scaling relations. However, the scaling schemes used in the literature differ. Aims. The present paper aims to clarify this situation by providing a thorough review of the scaling laws within the standard model of large-scale structure growth and to discuss various steps in practical approximations. Methods. We derived the scaling laws for X-ray observables and cluster mass within the pure gravitational structure growth scenario. Using N-body simulations we tested the recent formation approximation used in earlier analytic approaches. It involves a redshift-dependent overdensity parameter. We find this approximation less precise than using a fiducial radius based on a fixed overdensity with respect to critical density. Results. Inspired by the comparison of the predicted scaling relations with observations, we propose a first-order modification of the scaling scheme to include the observed effects of hydrodynamics in structure formation. This modification involves a cluster-mass dependent gas-mass fraction. We also discuss the observational results of the reshift evolution of the most important scaling relations and find that a redshift dependence of the gas mass to total mass relation also has to be invoked within our modification scheme. Conclusions. We find that the current observational data are, within their uncertainties, consistent with the proposed modified scaling laws.
We present a morphological study of 35 X-ray luminous galaxy clusters at 0.15 < z < 0.3, selected in a similar manner to the Local Cluster Substructure Survey (LoCuSS), for which deep XMM-Newton ...observations are available. We characterise the structure of the X-ray surface brightness distribution of each cluster by measuring both their power ratios and centroid shift, and thus rank the clusters by the degree of substructure. These complementary probes give a consistent description of the cluster morphologies with some well understood exceptions. We find a remarkably tight correlation of regular morphology with the occurrence of cool cores in clusters. We also compare our measurements of X-ray morphology with measurements of the luminosity gap statistics and ellipticity of the brightest cluster galaxy (BCG), to examine recent suggestions that these quantities may be efficient probes of the assembly history of and observer’s viewing angle through cluster-scale mass distributions. Our X-ray analysis confirms that cluster with large luminosity gaps form a relatively homogeneous population that appears more regular with the implication that such systems did not suffer from recent merger activity. Similarly, we find a clear correlation between the ellipticity of the BCGs and the shape of the cluster. In particular nearly circular BCGs (ϵ < 0.2) are found in undisturbed systems with regular X-ray morphologies. For these systems it has been suggested that they are intrinsically prolate and viewed along the line of sight close to the major axis. Finally, we check how our new X-ray morphological analysis maps onto cluster scaling relations, finding that (i) clusters with relatively undisturbed X-ray morphologies are on average more luminous at fixed X-ray temperature than those with disturbed morphologies, and (ii) disturbed clusters have larger X-ray masses than regular clusters for a given temperature in the MX − T relation. We also show that the scatter in the ratio of X-ray and weak lensing based cluster mass measurements is larger for disturbed clusters than for those of more regular morphology. Overall, our results demonstrate the feasibility of assembling a self-consistent picture of the physical structure of clusters from X-ray and optical data, and the potential to apply this in the measurement of cosmological cluster scaling relations.
Context. Galaxy clusters provide unique laboratories to study astrophysical processes on large scales and are important probes for cosmology. X-ray observations are currently the best means of ...detecting and characterizing galaxy clusters. Therefore X-ray surveys for galaxy clusters are one of the best ways to obtain a statistical census of the galaxy cluster population. Aims. In this paper we describe the construction of the REFLEX II galaxy cluster survey based on the southern part of the ROSAT All-Sky Survey. REFLEX II extends the REFLEX I survey by a factor of about two down to a flux limit of 1.8 × 10-12 erg s cm (0.1−2.4 keV). Methods. We describe the determination of the X-ray parameters, the process of X-ray source identification, and the construction of the survey selection function. Results. The REFLEX II cluster sample comprises currently 915 objects. A standard selection function is derived for a lower source count limit of 20 photons in addition to the flux limit. The median redshift of the sample is z = 0.102. Internal consistency checks and the comparison to several other galaxy cluster surveys imply that REFLEX II is better than 90% complete with a contamination less than 10%. Conclusions. With this publication we give a comprehensive statistical description of the REFLEX II survey and provide all the complementary information necessary for a proper modeling of the survey for astrophysical and cosmological applications.
Quadratic methods with heuristic weighting (e.g. pseudo-Cl 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-Cl 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 1 or 2 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 ∼0.05.
We report 19 intermediate redshift clusters newly detected in the ROSAT All-Sky survey that are spectroscopically confirmed. They form a part of 911 objects in the REFLEX II cluster catalogue with a ...limiting flux of 1.8 × 10-12 erg/s/cm2 in the 0.1–2.4 keV ROSAT band at redshift z ≥ 0.2. In addition we report three clusters from the REFLEX III supplementary catalogue, which contains objects below the REFLEX II flux limit but satisfies the redshift constraint above. These clusters are spectroscopically followed-up by our ESO NTT-EFOSC2 campaigns for the redshift measurement. We describe our observing and data reduction methods. We show how X-ray properties such as spectral hardness ratio and source extent can be used as important diagnostics in selecting galaxy cluster candidates. Physical properties of the clusters are subsequently calculated from the X-ray observations. This sample contains the high mass and intermediate-redshift galaxy clusters for astrophysical and cosmological applications.
We have identified new clusters and characterized previously unknown
Planck
Sunyaev–Zeldovich (SZ) sources from the first
Planck
catalogue of SZ sources (PSZ1). The results presented here correspond ...to an optical follow-up observational programme developed during approximately one year (2014) at Roque de los Muchachos Observatory, using the 2.5 m
Isaac Newton
telescope, the 3.5 m Telescopio Nazionale
Galileo
, the 4.2 m
William Herschel
telescope and the 10.4 m Gran Telescopio Canarias. We have characterized 115 new PSZ1 sources using deep optical imaging and spectroscopy. We adopted robust criteria in order to consolidate the SZ counterparts by analysing the optical richness, the 2D galaxy distribution, and velocity dispersions of clusters. Confirmed counterparts are considered to be validated if they are rich structures, well aligned with the
Planck
PSZ1 coordinate and show relatively high velocity dispersion. Following this classification, we confirm 53 clusters, which means that 46% of this PSZ1 subsample has been validated and characterized with this technique. Sixty-two SZ sources (54% of this PSZ1 subset) remain unconfirmed. In addition, we find that the fraction of unconfirmed clusters close to the galactic plane (at |
b
| < 25°) is greater than that at higher galactic latitudes (|
b
| > 25°), which indicates contamination produced by radio emission of galactic dust and gas clouds on these SZ detections. In fact, in the majority of the cases, we detect important galactic cirrus in the optical images, mainly in the SZ target located at low galactic latitudes, which supports this hypothesis.
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