Pasquale Mazzotta

We investigated the reconstruction of the intra-cluster medium radial density profiles in galaxy clusters, focusing on the impact of cluster morphology, dynamical state, and methodological choices, within the framework of the CHEX–MATE project. Using a sample of 109 simulated galaxy clusters analysed along 40 independent lines of sight, we compared azimuthal mean and median techniques for deprojection. We find that azimuthal median-based density profiles recover the true gas density with negligible bias over the radial range [0.15 − 1.3] R ₅₀₀ . Mean based profiles are affected by the presence of substructures. While an accurate masking of clumps or substructures can reduce the bias to < 5%, the discrepancy increases substantially in disturbed systems or when substructures remain unresolved. The mean-to-median density ratio statistically traces projected gas inhomogeneities, although intrinsic scatter and projection effects limit its use for individual clusters. The complementary use of mean and median profiles therefore provides the most complete approach for large X-ray cluster samples.

Measurements of thermodynamical quantities in galaxy clusters are differently affected by simplified modelling of radially averaged observables in the X-ray and millimetre bands. This includes assumptions about the cosmological model and the morphology of the cluster intracluster medium (ICM). Within a large sample of clusters extracted from T HE T HREE H UNDRED hydrodynamical simulations, we assess the systematic differences expected from the morphological assumptions between ICM temperatures as inferred from X-ray spectroscopy or joint X-ray and millimetre imaging. We find that these differences show a well-defined statistical behaviour that correlates with the cluster dynamical and morphological indicators. We then investigate how joint inferences of cluster temperature profiles, a priori informed by this statistical behaviour, allow us to constrain cosmological parameters inferred from the apparent cluster sizes. Assuming a flat Λ cold dark matter (ΛCDM) cosmology and priors on Ω _m and the helium abundance, this method provides us with unbiased estimates of the Hubble constant, H ₀ , characterised with a precision of about 4% and 1.5% for samples of 100 and 1000 clusters, respectively, and ultimately limited by systematic uncertainties of about 0.6–0.8 km s ⁻¹ Mpc ⁻¹ . This work highlights the potential of joint X-ray and millimetre observations of galaxy cluster samples to place tight constraints on H ₀ .

Context. Galaxy clusters offer powerful insights into the large-scale structure of the Universe and the physics of baryons in hot states. Their scientific exploitation, however, hinges on our ability to accurately measure key thermodynamic properties. Aims. In this work we assess the reliability of current analysis techniques in reconstructing these properties, with a particular focus on samples similar to those observed in the Cluster Heritage project with XMM-Newton (CHEX-MATE). Methods. We developed a suite of dedicated end-to-end simulations of CHEX-MATE-like clusters selected from large-scale hydrodynamical simulations and processed through a newly developed realistic XMM-Newton simulator. We applied a full X-ray data analysis pipeline that includes imaging, spectral fitting, and profile reconstruction to the mock datasets. Results. The gas density profiles can be robustly recovered across a wide radial range when using azimuthal mean surface brightness profiles. Our reconstruction techniques are able to reproduce the intrinsic density profile with the correct scatter, with deviations of at most 10% between 0.1 and 1 × R _500c . The gas mass is reconstructed with better than 1% accuracy. The accurate measurement of temperature profiles is more challenging and possibly subject to biases, particularly in the presence of azimuthal variations and multi-temperature gas along the line of sight, which dominate over projection effects. Conclusions. Our results highlight the need for caution when interpreting cluster temperature measurements and underscore the value of tailored mock observations for understanding observational systematics. These findings also suggest that biases in X-ray temperature measurements alter the interpretation of the thermodynamical state of the intra-cluster medium, an outlook particularly relevant in light of recent low-velocity measurements from the XRISM mission.

Under the standard model of hierarchical structure formation, the overall geometry of galaxy clusters is better described by a triaxial ellipse than by a sphere. As a result, the application of spherically symmetric models can result in significant biases, with masses derived from weak-lensing observations being particularly sensitive. These biases can be mitigated by fitting a triaxial model, but this requires deep multi-probe data along with a set of physically motivated models to describe them. We present a multi-probe triaxial analysis method based on the data available for galaxy clusters in the Cluster Heritage project with XMM-Newton – Mass Assembly and Thermodynamics at Endpoint of structure formation (CHEX-MATE), which includes X-ray data from XMM-Newton , Sunyaev-Zel’dovich effect maps from Planck and ACT, and weak-lensing data from Subaru. This work builds upon our previous development of a gas-only X-ray and Sunyaev-Zel’dovich triaxial fitting formalism in Paper I. After verifying our approach using mock observations of model clusters with known properties, we applied it to the CHEX-MATE galaxy cluster PSZ2 G313.33+61.13 (Abell 1689). We found that the cluster is elongated along the line of sight relative to the plane of sky by a factor of ℛ _LP = 1.20 ± 0.04. As a result, the weak-lensing mass obtained from our triaxial fit, M _{200 c} = (13.88 ^+1.73 _−1.43 ) × 10 ¹⁴ M _⊙ , is significantly lower than the value of (17.77 ^+2.00 _−1.75 ) × 10 ¹⁴ M _⊙ obtained from a spherically symmetric fit that otherwise employed the same method. Our triaxial fit finds a concentration of c _{200 c} = 8.66 ^+2.08 _−1.70 , consistent with the spherically symmetric value of 9.99 ^+2.26 _−1.8 , which suggests that the unexpectedly high concentration in Abell 1689 is not due to triaxiality and orientation. We also measured the nonthermal pressure fraction at radii between 0.18–1.37 Mpc and found a minimum of approximately 20% at intermediate radii, increasing to near 30% at the smallest and largest radii, and with a typical measurement precision of ±5%.

Thermodynamic profiles from X-ray and millimetre observations of galaxy clusters are often compared under the simplifying assumptions of a smooth, spherically symmetric intracluster medium. These approximations lead to expected discrepancies in the inferred profiles, which can provide insights into the cluster structure or cosmology. Motivated by this, we present a joint XMM- Newton and Planck analysis of 116 CHEX-MATE clusters to measure η _T = T _X / T _{SZ ,  X} , the ratio between spectroscopic X-ray temperatures and a temperature proxy derived from Sunyaev-Zel’dovich (SZ) pressures and X-ray densities. We considered relativistic corrections to the thermal SZ signal and implemented X-ray absorption by Galactic molecular hydrogen. The η _T distribution has a mean of 1.01 ± 0.03, with average changes of 8.1% and 2.7% when relativistic corrections and molecular hydrogen absorption are not included, respectively. The η _T distribution is positively skewed, with the scatter mostly affected by cluster morphology: relaxed clusters are closer to unity and less scattered than mixed and disturbed systems. We find little or no correlation with redshift, mass, or temperature.

In a self-similar paradigm of structure formation, the thermal pressure of the hot intra-cluster gas follows a universal distribution, once the profile of each cluster is normalised based on the proper mass and redshift dependencies. The reconstruction of such a universal pressure profile requires an individual estimate of the mass of each cluster. In this context, we present a method to jointly fit, for the first time, the universal pressure profile and individual cluster M ₅₀₀ masses over a sample of galaxy clusters, properly accounting for correlations between the profile shape and amplitude, and masses that scale the individual profiles. We demonstrate the power of the method and show that a consistent exploitation of the universal pressure profile and cluster mass estimates when modelling the thermal pressure in clusters is necessary to avoid biases. In particular, the method, informed by a cluster mass scale, outputs individual cluster masses with the same accuracy and greater precision than input masses. Using data from the Cluster HEritage project with XMM-Newton : Mass Assembly and Thermodynamics at the Endpoint of structure formation (CHEX-MATE), we investigate a sample of ∼25 galaxy clusters spanning mass and redshift ranges of 2 ≲ M ₅₀₀ /10 ¹⁴ M _⊙ ≲ 14 and 0.07 < z < 0.6.

The nature of the elusive dark matter can be probed by comparing the predictions of the cold dark matter framework with the gravitational field of massive galaxy clusters. However, a robust test of dark matter can only be achieved if the systematic uncertainties in the reconstruction of the gravitational potential are minimized. Techniques based on the properties of intracluster gas rely on the assumption that the gas is in hydrostatic equilibrium within the potential well, whereas gravitational lensing is sensitive to projection effects. Here we attempt to minimize systematics in galaxy cluster mass reconstructions by jointly exploiting the weak gravitational lensing signal and the properties of the hot intracluster gas determined from X-ray and millimeter (Sunyaev-Zel’dovich) observations. We constructed a model to fit the multiprobe information within a common framework, accounting for non-thermal pressure support and elongation of the dark matter halo along the line of sight. We then applied our framework to the massive cluster Abell 1689, which features unparalleled multiwavelength data. In accordance with previous works, we find that the cluster is significantly elongated along the line of sight. Accounting for line-of-sight projections, we require a non-thermal pressure support of 30 − 40% at r₅₀₀ to match the gas and weak lensing observables. The joint model retrieves a concentration c₂₀₀ ∼ 7, which is lower and better agrees with the concentration-mass relation than the high concentration retrieved from weak lensing data alone under the assumption of spherical symmetry (c₂₀₀ ∼ 15). Applying our method to a larger sample will allow us to study the shape of dark matter mass profiles and the level of non-thermal pressure support in galaxy clusters at the same time.

We characterise the intracluster gas entropy profiles of 32 very high-mass (M₅₀₀ > 7.75 × 10¹⁴ M_⊙) Planck SZ-detected galaxy clusters (HIGHMz), selected from the CHEX-MATE sample, allowing us to study the intracluster medium (ICM) entropy distribution in a regime where non-gravitational effects are expected to be minimised. Using XMM-Newton measurements, we determined the entropy profiles up to ∼R₅₀₀ for all objects. We assessed the relative role of gas density and temperature measurements on the uncertainty in entropy reconstruction, showing that in the outer regions the largest contribution comes from the temperature. The scaled profiles exhibit a large dispersion in the central regions, but converge rapidly to the value expected from simple gravitational collapse beyond the core regions. We quantified the correlation between the ICM morphological parameters and scaled entropy as a function of radius, showing that centrally peaked objects have low central entropy, while morphologically disturbed objects have high central entropy. We compared the scaled HIGHMz entropy profiles to results from other observational samples, finding differences in normalisation, which appear linked to the average mass of the samples in question. Combining HIGHMz with other samples, we found that a weaker mass dependence than self-similar in the scaling (A_m ∼ −0.25) allows us to minimise the dispersion in the radial range [0.3 − 0.8] R₅₀₀ for clusters spanning over a decade in mass. The deviation from self-similar predictions is radially dependent and is more pronounced at small and intermediate radii than at R₅₀₀. We also investigated the distribution of central entropy K₀, finding no evidence for bimodality in the data and outer slope α, which peaks at α ∼ 1.1 with tails at both low and high α that correlate with dynamical state. Using weak-lensing masses for half of the sample, we found an indication for a small suppression of the scatter (∼30%) beyond the core when using masses derived from Y_X in the rescaling. Finally, we compared our results to recent cosmological numerical simulations from THE THREE HUNDRED and MACSIS, finding good agreement with the observational data in this mass regime. These results provide a robust observational benchmark in the gravity-dominated regime, and will serve as a future reference for samples at lower masses, higher redshifts, and for ongoing work using cosmological numerical simulations.

Using Chandra observations, we derive the Y _X proxy and associated total mass measurement, , for 147 clusters with z < 0.35 from the Planck early Sunyaev–Zeldovich catalog, and for 80 clusters with z < 0.22 from an X-ray flux-limited sample. We reextract the Planck Y _SZ measurements and obtain the corresponding mass proxy, , from the full Planck mission maps, minimizing Malmquist bias due to observational scatter. The masses reextracted using the more precise X-ray position and characteristic size agree with the published PSZ2 values, but yield a significant reduction in the scatter (by a factor of two) in the – relation. The slope is 0.93 ± 0.03, and the median ratio, , is within the expectations from known X-ray calibration systematics. Y _SZ/Y _X is 0.88 ± 0.02, in good agreement with predictions from cluster structure, and implying a low level of clumpiness. In agreement with the findings of the Planck Collaboration, the slope of the Y _SZ– flux relation is significantly less than unity (0.89 ± 0.01). Using extensive simulations, we show that this result is not due to selection effects, intrinsic scatter, or covariance between quantities. We demonstrate analytically that changing the Y _SZ–Y _X relation from apparent flux to intrinsic properties results in a best-fit slope that is closer to unity and increases the dispersion about the relation. The redistribution resulting from this transformation implies that the best-fit parameters of the – relation will be sample-dependent.

We present the analysis of X-ray and optical observations of gas filaments observed in the radio source 3CR 318.1, associated with NGC 5920, the brightest cluster galaxy (BCG) of MKW 3 s, a nearby cool core galaxy cluster. This work is one of the first X-ray and optical analyses of filaments in cool core clusters carried out using MUSE observations. We aim at identifying the main excitation processes responsible for the emission arising from these filaments. We complemented the optical VLT/MUSE observations, tracing the colder gas phase, with X-ray Chandra observations of the hotter highly ionized gas phase. Using the MUSE observations, we studied the emission line intensity ratios along the filaments to constrain the physical processes driving the excitation, and, using the Chandra observations, we carried out a spectral analysis of the gas along these filaments. We found a spatial association between the X-ray and optical morphology of these filaments, which are colder and have lower metal abundance than the surrounding intracluster medium (ICM), as already seen in other BCGs. Comparing with previous results from the literature for other BCGs, we propose that the excitation process that is most likely responsible for these filaments emission is a combination of star formation and shocks, with a likely contribution from self-ionizing, cooling ICM. Additionally, we conclude that the filaments most likely originated from AGN-driven outflows in the direction of the radio jet.