arXiv subjects

S. De Grandi

Publications and source records attributed to S. De Grandi.

At least 19 recordsLinked to original sources

arXiv2026

The dynamics of the Anglerfish cluster

Merging galaxy clusters represent the ideal laboratory to test our understanding of the large scale structure formation history and the processes involved. While many merging clusters have been identified, only a limited number have been studied in detail through multi-wavelength analysis and dynamical reconstruction, this type of analysis being crucial to account for projection degeneracies. This work investigates the merger dynamics of the massive and complex cluster MACS0600 using high spatial, $\sim 15$ arcsec, radio and X-ray datasets in combination with ancillary optical data. We analyze the cluster morphology and the thermodynamic properties of the intracluster medium (ICM) through XMM-Newton and Chandra X-ray observations, and explore the non-thermal component via diffuse radio emission observed with Meerkat. We find a disturbed X-ray morphology with multiple substructures and a clear offset between the bulk of the radio emission and the X-ray peak. At the location of the X-ray peak, we detect a compact cool core surrounded by hotter gas and associated with a surface brightness discontinuity consistent with a cold front. The central region exhibits elevated temperatures and hosts most of the diffuse radio emission, suggesting merger-driven turbulence. Optical data further support a relative motion between the cool core and the main cluster along the line of sight. We conclude that MACS0600 is undergoing a merger in which a compact cool core has crossed the main, more massive cluster without being completely disrupted, while significantly perturbing the surrounding ICM.

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arXiv2026

CHEX-MATE: New detections and properties of the radio diffuse emission in massive clusters with MeerKAT

Modern radio telescopes are revolutionising our understanding of non-thermal phenomena in galaxy clusters, collecting large samples of extended sources with unprecedented sensitivity and angular resolution. In this work, we present novel MeerKAT observations for a sample of 21 galaxy clusters that are part of the CHEX-MATE project. These systems were selected based on their high mass and displaying signs of dynamical activity. Thanks to the high-quality data at hand, we were able to detect extended radio emission in every target considered. We report two new halos, one new relic, and two new candidate relics. We also confirm a previous candidate halo and two candidate relics. After investigating the scaling relations with the cluster properties, we confirmed the presence of a radio halo power-mass correlation and relate it to a higher radio halo emissivity in more massive clusters. For radio relics, we highlight the MeerKAT capabilities to significantly extend the depth of radio observations to a new, unexplored field of low-radio power sources ($\lesssim 10^{23} ~ {\rm W~Hz^{-1}} $ at 1.28 GHz). Thanks to such high-sensitivity data, we have found that the radio relic power can be characterised by a wide range of values for a given cluster mass and relic size. Ultimately, we discuss how current radio observations, in combination with large radio surveys, are increasingly capable of testing numerical simulation predictions and coming close to performing direct comparisons with their data, enabling new insights on the evolution of radio relics.

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arXiv2025

In medio stat virtus: enrichment history in poor galaxy clusters

The enrichment history of galaxy clusters and groups remains far from being fully understood. Recent measurements in massive clusters have revealed remarkably flat iron abundance profiles out to the outskirts, suggesting that similar enrichment processes have occurred for all systems. In contrast, abundance profiles in galaxy groups have sometimes been measured to decline with radius, challenging our understanding of the physical processes at these scales. In this paper, we present a pilot study aimed at accurately measuring the iron abundance profiles of MKW3s, A2589, and Hydra A, three poor clusters with total masses of $M_{500} \simeq 2.0-2.5 \times 10^{14}$ M$_\odot$, intermediate between the scales of galaxy groups and massive clusters. Using XMM-Newton to obtain nearly complete azimuthal coverage of the outer regions of these systems, we show that abundance measurements in the outskirts are more likely to be limited by systematics than by statistical errors. In particular, inaccurate modelling of the soft X-ray background can significantly bias metallicity estimates in regions where the cluster emission is faint. Once these systematics are properly accounted for, the abundance profiles of all three clusters appear to be flat at $Z \sim 0.3$ Z$_{\odot}$, in agreement with values observed in massive clusters. Using available stellar mass estimates, we also computed their iron yields, thereby beginning to probe a largely unexplored mass range. We find $Y_{Fe,500} = 2.68\pm0.34$, $2.54\pm0.64$, and $7.51\pm1.47$ Z$_{\odot}$ for MKW3s, A2589, and Hydra A, respectively, spanning the transition regime between galaxy groups and massive clusters. Future observations of systems with temperatures of $2-4$ keV will be essential to further populate this intermediate-mass regime and to draw firmer conclusions on the chemical enrichment history of galaxy systems across the full mass scale.

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arXiv2025

Understanding entropy in massive halos: The role of baryon decoupling

The goal of the work presented in this paper is to use observed entropy profiles to infer constraints on the accretion process in massive halos. We compare entropy profiles from various observational samples with those generated by an updated version of the semi-analytical models developed in the early 2000s, modified to reflect recent advancements in our understanding of large-structure formation. Our model reproduces the growing departure from self-similarity observed in data as we move inward in individual profiles and down in mass across different profiles. These deviations stem from a phase of extremely low gas content centered around $10^{13}$M$_\odot$. According to our model, halos at this mass scale are missing between 50% and 90% of their baryons, corresponding to a gas fraction ranging between 2% and 8%. Baryon decoupling, the mechanism at the heart of our model, proves effective in explaining much of the behavior we sought to understand.

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arXiv2024

Spherical bias on the 3D reconstruction of the ICM density profile in galaxy clusters

X-ray observations of galaxy clusters are routinely used to derive radial distributions of ICM thermdynamical properties such as density and temperature. However, observations allow us to access quantities projected on the celestial sphere only, so that an assumption on the 3D distribution of the ICM is necessary. Usually, spherical geometry is assumed. The aim of this paper is to determine the bias due to this approximation on the reconstruction of ICM density radial profile of a clusters sample and on the intrinsic scatter of the density profiles distribution, when clusters substructures are not masked. We used 98 simulated clusters for which we know the 3D ICM distribution drawn from The Three Hundred project. For each cluster we simulated 40 different observations by projecting the cluster along 40 different lines of sight. We extracted the ICM density profile from each observation assuming the ICM to be spherical distributed. For each line of sight we then considered the mean density profile over the sample and compared it with the 3D density profile given by the simulations. The spherical bias on the density profile is derived by considering the ratio between the observed and the input quantities. We also study the bias on the intrinsic scatter of the density profile distribution performing the same procedure. We find a bias on the density profile, $b_n$, smaller than $10\%$ for $R\lesssim R_{500}$ while it increases up to $\sim 50\%$ for larger radii. The bias on the intrinsic scatter profile, $b_s$, reaches a value of $\approx 100\%$ for $R\approx R_{500}$. The bias on both the analysed quantities strongly depends on the morphology of the objects: for clusters that do not show large scale substructures, both $b_n$ and $b_s$ are reduced by a factor 2, conversely for systems that do show large scale substructures both $b_n$ and $b_s$ increase significantly. [abridged]

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arXiv2024

CHEX-MATE: the intracluster medium entropy distribution in the gravity-dominated regime

We characterise the entropy profiles of 32 very high mass ($M_{500}>7.75\times10^{14}~M_{\odot}$) galaxy clusters (HIGHMz), selected from the CHEX-MATE sample, to study the intracluster medium (ICM) entropy distribution in a regime where non-gravitational effects are minimised. Using XMM-Newton measurements, we measure the entropy profiles up to ~$R_{500}$ for all objects. The scaled profiles exhibit large dispersion in the central regions, but converge rapidly to the expectation from pure gravitational collapse beyond the core. We quantify the correlation between the ICM morphological parameters and scaled entropy as a function of radius, showing that morphologically relaxed (disturbed) objects have low (high) central entropy. We compare our data to other observational samples, finding differences in normalisation which are linked to the average mass of the samples in question. We find that a weaker mass dependence than self-similar in the scaling (Am ~ -0.25) allows us to minimise the dispersion in the radial range [0.3-0.8]$R_{500}$ for clusters spanning over a decade in mass. The deviation from self-similarity is radially dependent and is more pronounced at small and intermediate radii than at $R_{500}$. We also investigate the distribution of central entropy $K_0$, finding no evidence for bimodality, and outer slopes $α$, which peaks at ~1.1. Using weak lensing masses, we find indication for a small suppression of the scatter (~30%) beyond the core when using masses derived from Yx in the rescaling. Finally, we compare to recent cosmological numerical simulations from THE THREE HUNDRED and MACSIS, finding good agreement with our observational data. These results provide a robust observational benchmark in the gravity-dominated regime and will serve as a future reference for samples at lower mass, higher redshifts, and for ongoing work using cosmological numerical simulations.

astro-ph.CO↗

arXiv2024

PSZ2 G282.28+49.94, a recently discovered analogue of the famous Bullet Cluster

We present a detailed study of the gas and galaxy properties of the cluster PSZ2 G282.28+49.94 detected in the Planck all-sky survey. The intracluster medium (ICM) of this object at z=0.56 exhibits a cometary-like shape. Combining Chandra and TNG observations, we characterised the spatially resolved thermodynamical properties of the gas and the spatial and velocity distribution of 73 galaxy members. The cluster structure is quite complex with an elongated core region containing the two brightest cluster galaxies and one dense group to the south-east. Since there is no velocity difference between the core and the south-east group, we suggest the presence of a merger along the plane of the sky. This structure is related to complex X-ray and radio features, and thus the merger has likely been caught during the post-merger phase. Comparing the distribution of the ICM and of member galaxies, we find a large offset of $\sim 350$ kpc between the position of the X-ray peak and the centre of a concentration of galaxies, preceding it in the likely direction of motion. This configuration is similar to the famous Bullet Cluster, leading us to dub PSZ2 G282.28+49.94 the "Planck bullet", and represents an ideal situation to provide astrophysical constraints to the self-interaction cross-section ($σ/m$) of dark matter particles. These results illustrate the power of a multi-wavelength approach to probe the merging scenario of such complex and distant systems.

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arXiv2024

Metal enrichment: the apex accretor perspective

Aims. The goal of this work is to devise a description of the enrichment process in large-scale structure that explains the available observations and makes predictions for future measurements. Methods. We took a spartan approach to this study, employing observational results and algebra to connect stellar assembly in star-forming halos with metal enrichment of the intra-cluster and group medium. Results. On one hand, our construct is the first to provide an explanation for much of the phenomenology of metal enrichment in clusters and groups. It sheds light on the lack of redshift evolution in metal abundance, as well as the small scatter of metal abundance profiles, the entropy versus abundance anti-correlation found in cool core clusters, and the so-called Fe conundrum, along with several other aspects of cluster enrichment. On the other hand, it also allows us to infer the properties of other constituents of large-scale structure. We find that gas that is not bound to halos must have a metal abundance similar to that of the ICM and only about one-seventh to one-third of the Fe in the Universe is locked in stars. A comparable amount is found in gas in groups and clusters and, lastly and most importantly, about three-fifths of the total Fe is contained in a tenuous warm or hot gaseous medium in or between galaxies. We point out that several of our results follow from two critical but well motivated assumptions: 1) the stellar mass in massive halos is currently underestimated and 2) the adopted Fe yield is only marginally consistent with predictions from synthesis models and SN rates. Conclusions. One of the most appealing features of the work presented here is that it provides an observationally grounded construct where vital questions on chemical enrichment in the large-scale structure can be addressed. We hope that it may serve as a useful baseline for future works.

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arXiv2024

CHEX-MATE: A LOFAR pilot X-ray$-$radio study on five radio halo clusters

The connection between the thermal and non-thermal properties in galaxy clusters hosting radio halos seems fairly well established. However, a comprehensive analysis of such a connection has been made only for integrated quantities (e.g. $L_X - P_{radio}$ relation). In recent years new-generation radio telescopes have enabled the unprecedented possibility to study the non-thermal properties of galaxy clusters on a spatially resolved basis. Here, we perform a pilot study to investigate the mentioned properties on five targets, by combining X-ray data from the CHEX-MATE project with the second data release from the LOFAR Two meter Sky survey. We find a strong correlation ($r_s \sim 0.7$) with a slope less than unity between the radio and X-ray surface brightness. We also report differences in the spatially resolved properties of the radio emission in clusters which show different levels of dynamical disturbance. In particular, less perturbed clusters (according to X-ray parameters) show peaked radio profiles in the centre, with a flattening in the outer regions, while the three dynamically disturbed clusters have steeper profiles in the outer regions. We fit a model to the radio emission in the context of turbulent re-acceleration with a constant ratio between thermal and non-thermal particles energy density and a magnetic field profile linked to the thermal gas density as $B(r) \propto n_{th}^{0.5}$. We found that this simple model cannot reproduce the behaviour of the observed radio emission.

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arXiv2024

CHEX-MATE: Robust reconstruction of temperature profiles in galaxy clusters with XMM-Newton

The "Cluster HEritage project with \xmm: Mass Assembly and Thermodynamics at the Endpoint of structure formation" (CHEX-MATE) is a multi-year Heritage program, to obtain homogeneous XMM-Newton observations of a representative sample of 118 galaxy clusters. The observations are tuned to reconstruct the distribution of the main thermodynamic quantities of the ICM up to $R_{500}$ and to obtain individual mass measurements, via the hydrostatic-equilibrium equation, with a precision of 15-20%. Temperature profiles are a necessary ingredient for the scientific goals of the project and it is thus crucial to derive the best possible temperature measurements from our data. This is why we have built a new pipeline for spectral extraction and analysis of XMM-Newton data, based on a new physically motivated background model and on a Bayesian approach with Markov Chain Monte Carlo (MCMC) methods, that we present in this paper for the first time. We applied this new method to a subset of 30 galaxy clusters representative of the CHEX-MATE sample and show that we can obtain reliable temperature measurements up to regions where the source intensity is as low as 20% of the background, keeping systematic errors below 10%. We compare the median profile of our sample and the best fit slope at large radii with literature results and we find a good agreement with other measurements based on XMM-Newton data. Conversely, when we exclude from our analysis the most contaminated regions, where the source intensity is below 20 of the background, we find significantly flatter profiles, in agreement with predictions from numerical simulations and independent measurements with a combination of Sunyaev-Zeldovich and X-ray imaging data.

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arXiv2023

CHEX-MATE: Characterization of the intra-cluster medium temperature distribution

We study the perturbations in the temperature (and density) distribution for 28 clusters selected from the CHEX-MATE sample to evaluate and characterize the level of inhomogeneities and the related dynamical state of the ICM. We use these spatially resolved 2D distributions to measure the global and radial scatter and identify the regions that deviate the most from the average distribution. During this process, we introduce three dynamical state estimators and produce clean temperature profiles after removing the most deviant regions. We find that the temperature distribution of most of the clusters is skewed towards high temperatures and is well described by a log-normal function. There is no indication that the number of regions deviating more than 1$σ$ from the azimuthal value is correlated with the dynamical state inferred from morphological estimators. The removal of these regions leads to local temperature variations up to 10-20% and an average increase of $\sim$5% in the overall cluster temperatures. The measured relative intrinsic scatter within $R_{500}$, $σ_{T,int}/T$, has values of 0.17$^{+0.08}_{-0.05}$, and is almost independent of the cluster mass and dynamical state. Comparing the scatter of temperature and density profiles to hydrodynamic simulations, we constrain the average Mach number regime of the sample to $M_{3D}$=0.36$^{+0.16}_{-0.09}$. We infer the ratio between the energy in turbulence and the thermal energy, and translate this ratio in terms of a predicted hydrostatic mass bias $b$, estimating an average value of $b\sim$0.11 (covering a range between 0 and 0.37) within $R_{500}$. This study provides detailed temperature fluctuation measurements for 28 CHEX-MATE clusters which can be used to study turbulence, derive the mass bias, and make predictions on the scaling relation properties.

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arXiv2023

A three-component giant radio halo: the puzzling case of the galaxy cluster Abell 2142

Turbulence introduced into the intra-cluster medium (ICM) through cluster merger events transfers energy to non-thermal components, and can trigger the formation of diffuse synchrotron radio sources. Typical diffuse sources in the forms of giant radio halos and mini-halos are found in merging and relaxed cool core galaxy clusters, respectively. On the other hand, recent observations have revealed an increasing complexity of the non-thermal phenomenology. Abell 2142 (A2142) is a mildly disturbed cluster that exhibits uncommon thermal and non-thermal properties. It is known to host a hybrid halo consisting of two components (H1 and H2), namely a mini-halo-like and an enigmatic elongated radio halo-like structure. We aim to investigate the properties, origin, and connections of each component. We present deep LOFAR observations of A2142 in the frequency ranges $30-78$ MHz and $120-168$ MHz. With complementary multi-frequency radio and X-ray data, we analyse the radio spectral properties of the halo and assess the connection between the non-thermal and thermal components of the ICM. We detected a third radio component (H3), which extends over the cluster volume on scales $\sim 2$ Mpc, embeds H1 and H2, and has a morphology that roughly follows the thermal ICM distribution. The radio spectral index is moderately steep in H1 ($α=1.09\pm 0.02$) and H2 ($α=1.15\pm 0.02$), but is steeper ($α=1.57\pm 0.20$) in H3. The analysis of the thermal and non-thermal properties allowed us to discuss possible formation scenarios for each radio component. Turbulence from sloshing motions of low-entropy gas on different scales may be responsible for the origin of H1 and H2. We classified H3 as a giant ultra-steep spectrum radio halo, which could trace the residual activity from an old energetic merger and/or inefficient turbulent re-acceleration induced by ongoing minor mergers.

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arXiv2023

CHEX-MATE: Constraining the origin of the scatter in galaxy cluster radial X-ray surface brightness profiles

We investigate the statistical properties and the origin of the scatter within the spatially resolved surface brightness profiles of the CHEX-MATE sample, formed by 118 galaxy clusters selected via the SZ effect. These objects have been drawn from the Planck SZ catalogue and cover a wide range of masses, M$_{500}=[2-15] \times 10^{14} $M$_{\odot}$, and redshift, z=[0.05,0.6]. We derived the surface brightness and emission measure profiles and determined the statistical properties of the full sample. We found that there is a critical scale, R$\sim 0.4 R_{500}$, within which morphologically relaxed and disturbed object profiles diverge. The median of each sub-sample differs by a factor of $\sim 10$ at $0.05\,R_{500}$. There are no significant differences between mass- and redshift-selected sub-samples once proper scaling is applied. We compare CHEX-MATE with a sample of 115 clusters drawn from the The Three Hundred suite of cosmological simulations. We found that simulated emission measure profiles are systematically steeper than those of observations. For the first time, the simulations were used to break down the components causing the scatter between the profiles. We investigated the behaviour of the scatter due to object-by-object variation. We found that the high scatter, approximately 110%, at $R<0.4R_{500}$ is due to a genuine difference between the distribution of the gas in the core. The intermediate scale, $R_{500} =[0.4-0.8]$, is characterised by the minimum value of the scatter on the order of 0.56, indicating a region where cluster profiles are the closest to the self-similar regime. Larger scales are characterised by increasing scatter due to the complex spatial distribution of the gas. Also for the first time, we verify that the scatter due to projection effects is smaller than the scatter due to genuine object-by-object variation in all the considered scales. [abridged]

astro-ph.CO↗

arXiv2023

Optical/X-ray/radio view of Abell 1213: A galaxy cluster with anomalous diffuse radio emission

Context. Abell 1213, a low-richness galaxy system, is known to host an anomalous radio halo detected in data of the VLA. It is an outlier with regard to the relation between the radio halo power and the X-ray luminosity of the parent clusters. Aims. Our aim is to analyze the cluster in the optical, X-ray, and radio bands to characterize the environment of its diffuse radio emission and to shed new light on its nature. Methods. We used optical data from the SDSS to study the internal dynamics of the cluster. We also analyzed archival XMM-Newton X-ray data to unveil the properties of its hot intracluster medium. Finally, we used recent data from LOFAR at 144 MHz, together with VLA data at 1.4 GHz, to study the spectral behavior of the diffuse radio source. Results. Both our optical and X-ray analysis reveal that this low-mass cluster exhibits disturbed dynamics. In fact, it is composed of several galaxy groups in the peripheral regions and, in particular, in the core, where we find evidence of substructures oriented in the NE-SW direction, with hints of a merger nearly along the line of sight. The analysis of the X-ray emission adds further evidence that the cluster is in an unrelaxed dynamical state. At radio wavelengths, the LOFAR data show that the diffuse emission is ~510 kpc in size. Moreover, there are hints of low-surface-brightness emission permeating the cluster center. Conclusions. The environment of the diffuse radio emission is not what we would expect for a classical halo. The spectral index map of the radio source is compatible with a relic interpretation, possibly due to a merger in the N-S or NE-SW directions, in agreement with the substructures detected through the optical analysis. The fragmented, diffuse radio emissions at the cluster center could be attributed to the surface brightness peaks of a faint central radio halo.

astro-ph.CO↗

arXiv2022

The evolving cluster cores: Putting together the pieces of the puzzle

In this work we address the issue of whether the division of clusters in cool cores (CCs) and non-cool cores (NCCs) is due to a primordial difference or to how clusters evolve across cosmic time. Our first goal is to establish if spectra from the central regions of a subclass of NCCs known as cool core remnants (CCRs) are consistent with having a small but significant amount of short cooling time gas, thereby allowing a transformation to CC systems on a timescale of a giga year. Our second goal is to determine if low ionization Fe lines emitted from this residual cool gas will be detectable by the calorimeters that will fly on board XRISM and ATHENA. We performed a spectral analysis of CCR systems with a multi temperature model and, assuming the different components to be in pressure equilibrium with one another, derived entropy and cooling time distributions for the X-ray emitting gas. We find that in most of our systems, the spectral model allows for a fraction of low entropy, short cooling time gas with a mass that is comparable to the one in CC systems. Moreover, simulations show that future spectrometers on board XRISM and ATHENA will have the power to directly resolve emission lines from the low temperature gas, thereby providing incontrovertible evidence for its presence. Within the scenario that we have explored, the constant fraction of CCs measured across cosmic time emerges from a dynamical equilibrium where CCs transformed in NCCs through mergers are balanced by NCCs that revert to CCs. Furthermore, CCs and NCCs should not be viewed as distinct sub classes, but as ``states" between which clusters can move.

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arXiv2022

Does the Fe L-shell blend bias abundance measurements in intermediate-temperature clusters?

In intermediate-temperature (T = 2.5 - 4.5 keV) galaxy clusters, abundance measurements are almost-equally driven by Fe K and L transitions, at $\sim$ 6.7 keV and 0.9 - 1.3 keV, respectively. While K-shell-derived measurements are considered reliable, the resolution of the currently available instrumentation, as well as our current knowledge of the atomic processes, makes the modelling of the L-line complex challenging, resulting in potential biases for abundance measurements. In this work, we study systematics related to the modelling of the Fe L-line complex that may influence iron-abundance measurements in the intermediate-temperature range. To this aim, we select a sample of three bright galaxy clusters, with long XMM-Newton observations available and temperature in the 2.5 - 4.5 keV range. We fit spectra extracted from concentric rings with APEC and APEC+APEC models, by alternatively excluding the L and K bands, and derive the fractional difference of the metal abundances, $ΔZ/Z$, as indication of the consistency between K- and L-shell-derived measurements. The $ΔZ/Z$ distribution is then studied as a function of the cluster radius, ring temperature and X-ray flux. The L-induced systematics, measured through an individual fit of each MOS and pn spectrum, remain constant at a 5 - 6% value in the whole 2.5 - 4.5 keV temperature range. Conversely, a joint fit of MOS and pn spectra leads to a slight excess of 1 - 2% in the above estimate. No significant dependence on the ring X-ray flux is highlighted. The measured 5 - 8% value indicates a modest contribution of the systematics to the derived iron abundances, giving confidence for future measurements. To date, these findings represent the best-achievable estimate of the systematics in analysis, while future microcalorimeters will significantly improve our understanding of the atomic processes underlying the Fe L emissions.

astro-ph.CO↗

arXiv2021

Accretion processes in the galaxy cluster Hydra A/Abell 780

Clusters of galaxies evolve and accrete mass, mostly from small galaxy systems. Our aim is to study the velocity field of the galaxy cluster Abell 780, which is known for the powerful radio source Hydra A at its center and where a spectacular X-ray tail associated with the galaxy LEDA 87445 has been discovered. Our analysis is based on the new spectroscopic data for hundreds of galaxies obtained with the Italian Telescopio Nazionale {\em Galileo} and the Very Large Telescope. We have constructed a redshift catalog of 623 galaxies and selected a sample of 126 cluster members. We analyze the internal structure of the cluster using a number of techniques. We estimate the mean redshift z=0.0545, the line-of-sight velocity dispersion sigmav about 800 km/s, and the dynamical mass M200 about 5.4 10E14 solar masses. The global properties of Abell 780 are typical of relaxed clusters. On a smaller scale, we can detect the presence of a galaxy group associated with LEDA 87445 in projected phase space. The mean velocity and position of the center of the group agree well with the velocity and position of LEDA 87445. We estimate the following parameters of the collision. The group is characterized by a higher velocity relative to the main system. It is infalling at a rest frame velocity of Vrf=+870 km/s and lies at a projected distance D=1.1 Mpc to the south, slightly southeast of the cluster center. The mass ratio between the group and the cluster is about 1:5. We also find evidence of an asymmetry in the velocity distribution of galaxies in the inner cluster region, which might be related to a small low-velocity group detected as a substructure at Vrf=-750 km/s. We conclude that A780, although dynamically relaxed at first sight, contains small substructures that may have some impact on the energetics of the core region.

astro-ph.CO↗

arXiv2021

X-ray characterisation of the massive galaxy clusterClG-J104803.7+313843 at z=0.76 with XMM-Newton

We present the characterisation of the massive cluster ClG-J$104803.7+313843$ at $z=0.76$ performed using a serendipitous XMM-Newton observation. High redshift and massive objects represent an ideal laboratory to benchmark our understanding of how cluster form and assembly formation driven mainly by gravity.Leveraging the high throughput of XMM-Newton we were firstly able to determine the redshift of the object, shedding light on ambiguous photometric redshift associations. We investigated the morphology of this cluster which shows signs of merging activities in the outskirts and a flat core. We also measured the radial density profile up to $R_{500}$. With these quantities in hand, we were able to determine the mass, $M_{500}=5.64^{+0.79}_{-0.62} \times 10^{14}M_{\odot}$, using the YX proxy. This quantity improves previous measurement of the mass of this object by a factor of $\sim 3.5$. The characterisation of one cluster at such mass and redshift regime is fundamental as these objects are intrinsically rare, the number of objects discovered so far being less than $\sim 25$. Our study highlights the importance of using X-ray observations in combination with ancillary multi-wavelength data to improve our understanding of high-z and massive clusters

astro-ph.CO↗