Clusters of galaxies are the largest and most massive, gravitational bound structures in the Universe.
Lying at the nodes of the cosmic web filaments in the present highly structured Universe, clusters of galaxies are at crossroads of cosmology and astrophysics: on one side they trace cosmic evolution providing information on structure formation, and are powerful tools to test the underlying cosmological models; on the other side they are unique astrophysical laboratories, ideal for the study of plasma physics, thermal and non-thermal processes, turbulence, magnetic fields, dark matter, etc.
Clusters are complex structures. They typically contain from tens to thousands galaxies. Nonetheless only few percent of their total mass is in the form of optical galaxies. Dark matter is the dominant (about 80%) component of clusters. About 15-20% of the total mass consists of diffuse, hot, metal-enriched, X-ray emitting plasma permeating the space between the galaxies: the intracluster medium (ICM).
Our group is active in exploiting data from various X-ray satellites (XMM-Newton, Chandra, NuSTAR, XRISM) to inspect in details clusters’ physics and evolution, aided also by observations at other wavelengths: radio (e.g. MeerKAT, LOFAR), millimeter (through the SZ effect, e.g. Planck and Nika2) and optical (e.g. VST, HST and JWST). We are involved in several projects like CHEX-MATE, X-GAP, FornaX, VST-GAME, LSST and we actively contribute to the development of new X-ray telescopes such as NewAthena.
A list of thesis projects is available here.
Thermodynamical properties of the ICM
The Intracluster Medium (ICM) is the dominant baryonic component of galaxy clusters. While it is often approximated as a fluid in hydrostatic equilibrium within the dark matter potential well, its true thermodynamic state is far more complex. The spatial distribution of its temperature, density, entropy, and pressure reflects not only the total cluster mass but is also heavily shaped by the system’s assembly history and non-gravitational processes, such as AGN feedback.
Using representative cluster samples like CHEX-MATE, we want to address these open questions:
- To what extent do observed thermodynamic profiles deviate from purely gravitational predictions? Do these deviations correlate with cluster mass, redshift, or dynamical state?
- How does the selection method of a sample bias the observed distribution of thermodynamic profiles, and what are the properties of the true, underlying cluster population?
- Can we accurately map thermodynamics in the faint cluster outskirts—where the bulk of cosmic accretion occurs—despite the dominance of background components over the cluster emission?
Astrophysics with galaxy groups
Galaxy groups are key laboratories for studying the interplay between gravitational assembly and baryonic physics. In these low-mass systems, the energy injected by supernovae and AGN feedback can be comparable to or exceed the gravitational binding energy, making non-gravitational processes a dominant driver of the thermodynamic state of the intragroup medium (IGrM). As a result, groups are not simply scaled-down clusters, but systems where cooling, feedback, and gas removal strongly reshape the baryon distribution and the scaling relations. Despite their importance for astrophysical and cosmological studies, groups remain observationally challenging due to their low surface brightness and small galaxy populations, leading to incomplete and heterogeneous samples. Ongoing and upcoming multi-wavelength surveys (X-ray in particular with eROSITA, optical, SZ, and radio) will dramatically increase the number of detected systems, but also amplify the need for robust selection functions and mass proxies. Simulations, combined with forward modeling of observables, will be essential to interpret these samples and quantify biases. Using representative optically and X-ray selected samples such as XGAP we want to address a list of open questions:
- How complete are current group samples, and what populations remain undetected (e.g. low surface brightness or gas-poor systems)?
- How can we reliably calibrate group masses across X-ray, optical, SZ, and weak lensing methods?
- What fraction of baryons is missing in groups, and where are they located (e.g. expelled, cooled, or redistributed)?
- How do selection effects impact observed scaling relations and inferred feedback efficiencies?
- How, and by how much, the non-gravitational processes are impacting the radial properties of groups?
- How do different AGN feedback modes (thermal vs kinetic vs radiative) impact ICM entropy and gas fractions?
Non-thermal phenomena in the ICM
Galaxy clusters are massive laboratories for studying the complex interplay between thermal gas, relativistic particles, and magnetic fields. While the bulk of the intra-cluster medium (ICM) consists of hot, X-ray emitting thermal plasma, non-thermal components, namely cosmic ray electrons (CRe) and microgauss-scale magnetic fields, permeate the cluster volume. These components are uniquely revealed across Megaparsec scales by diffuse synchrotron emission, manifesting as centralized radio halos, peripheral radio relics, and core-bound mini-halos. These structures are directly linked to cluster mergers and accretion events, which drive massive shocks and turbulence capable of accelerating or re-accelerating particles.
Mapping the full population of these faint, diffuse structures has been observationally challenging due to their incredibly low surface brightness and the difficulty of disentangling them from discrete radio galaxies. High-sensitivity, low-frequency radio surveys (such as LOFAR and MeerKAT) are dramatically expanding the known sample of non-thermal sources. Synergies with the analysis of multi-wavelength data (X-ray, Gamma-ray, and Sunyaev-Zel’dovich effect), will be essential to interpret these new data and constrain the underlying plasma microphysics.
A list of open questions we are trying to answer is:
- What are the primary mechanisms driving particle acceleration?
- How are magnetic fields amplified and structured across the ICM?
- What is radial distribution of the magnetic field from the dense core to the outskirts and its evolution with redshift ?
- What fraction of the total cluster pressure is held by non-thermal components?
Metal enrichment of the ICM
The hot plasma that constitutes the intracluster medium (ICM) and the intragroup medium (IGrM) mainly contains ionized hydrogen and helium but is also rich of heavier elements (C, N, O, Si, S, Fe etc…) commonly referred to as metals. Metals have been synthesized over cosmic time within stars, primarily in supernovae (SN) and then ejected into the surrounding environment through SN explosions and stellar winds. As such, the chemical composition of the ICM and IGrM encodes crucial information on the integrated history of star formation, while the spatial distribution of metals in these systems is strictly connected to their dynamical history and to the feedback mechanisms of active galactic nuclei (AGNs), outflows or jets which inject and spread the metal content in the surrounding environment.
Metal abundance has been routinely measured with the advent of the modern X-ray telescopes, XMM-Newton, Chandra, and Suzaku. We are now entering a new era for X-ray astronomy, where high-resolution X-ray spectroscopy, driven by the recent launch of XRISM and the future NewAthena mission, will provide unprecedented insights into metal abundances and the history of cosmic chemical enrichment. Exploiting X-ray observations, we want to address these open questions:
- When, where, and how did the chemical enrichment occur?
- Are abundance profiles in groups different from those in massive clusters?
- What’s the global budget of the various chemical elements in clusters and groups? And how is it related to the stellar content in these systems?
- What’s the role of the central BCG (Brightest Cluster Galaxy) in shaping the metal profile in the core of groups and clusters?
Mass estimates and scaling relations of galaxy clusters and groups
Determining total cluster mass, dominated by dark matter, is essential for both understanding cluster physics and constraining cosmological parameters. To robustly anchor the absolute mass scale, a multi-wavelength approach combining independent methodologies like X-ray hydrostatic masses, gravitational lensing, and galaxy dynamics is crucial.
The accuracy of mass estimates derived under the assumption of hydrostatic equilibrium is a major focus of modern research. Coherent and turbulent gas motions can provide significant non-thermal pressure support, inducing deviations from equilibrium that bias mass estimates downward and systematically affect cluster-based cosmological constraints. Multi-wavelength programs such as CHEX-MATE provide the ideal framework to calibrate this mass scale, while high-resolution X-ray spectroscopy, now with XRISM and in the future with NewAthena, is finally allowing us to directly measure the velocity dispersion of the gas and quantify this non-thermal contribution.
When expensive, multi-method data are unavailable, scaling relations serve as an indispensable tool to estimate masses across large-scale cosmic surveys. These relations link total cluster mass to easily accessible, multi-wavelength proxies or connect different observables to one another. Rooted in the self-similar collapse model of structure formation, deviations from simple gravitational scaling laws provide a powerful diagnostic of the energetic impact of non-gravitational physics, such as AGN and supernova feedback.
Our group investigates the systematic uncertainties and evolutionary trends in cluster masses through these lines of inquiry:
- The Hydrostatic Mass Bias: What is the exact magnitude of the hydrostatic mass bias, and does it vary systematically based on total mass, redshift, dynamical state, or the survey selection method?
- Turbulent Pressure Support: How much do turbulent and bulk gas motions contribute to the total pressure budget of the cluster, and how can we best correct for the resulting mass bias?
- Gas Fractions & Cosmological Baryons: What is the gas mass fraction in clusters and groups, how does it scale with system mass, and how does it compare to the cosmic baryon fraction?
- Evolution of Scaling Relations: How do mass-observable and observable-observable scaling relations evolve as a function of mass and redshift? What do deviations from theoretical self-similarity reveal about the impact of non-gravitational processes?
Cosmic Filaments in Large-Scale Structure
The filaments of the cosmic web form the large-scale backbone of the Universe, channeling matter into massive nodes like galaxy clusters. Cosmological simulations predict that a significant fraction of the “missing baryons” at low redshift reside within these vast intergalactic bridges in the form of a hot, diffuse plasma known as the Warm-Hot Intergalactic Medium (WHIM). Near galaxy clusters, the densest parts of these filaments are sufficiently compressed and shock-heated to be detected through faint soft X-ray emission and highly ionized oxygen lines (O VII and O VIII). Conversely, the far more common low-density filaments crossing cosmic voids are too faint to observe in emission and can only be studied through X-ray absorption spectroscopy against bright background sources such as active galactic nuclei (AGNs) or the rapidly fading afterglows of Gamma-Ray Bursts (GRBs).
Mapping the cosmic web in X-rays represents an extraordinary observational challenge. The WHIM surface brightness rapidly decreases away from cluster cores, requiring highly sensitive instruments and meticulous subtraction of contaminating point-sources. In absorption, the expected oxygen lines are very weak making unambiguous detections rare and heavily prone to confusion with the local interstellar or circumgalactic medium. Future observatories equipped with high-resolution microcalorimeters, such as the X-IFU onboard NewAthena, will be crucial for overcoming these limitations.
A list of open questions we are trying to answer is:
- What is the exact thermal and ionization state of the WHIM within different filament environments?
- How can we cleanly disentangle intrinsic host-galaxy or Galactic absorption from true cosmic web filaments?
- What fraction of the missing baryons is distributed in the diffuse cosmic web versus the circumgalactic medium (CGM)?
- What role do magnetic fields and cosmic rays play in filament dynamics and accretion of gas onto cluster nodes?
Galaxy evolution in cluster environment
Galaxy clusters are ideal astrophysical laboratories for investigating the impact of extreme environments on galaxy evolution. In these massive systems, mechanisms like ram pressure stripping, mergers, and tidal harassment rapidly transform star-forming, late-type galaxies into passive, early-type populations.
Because many clusters are still actively assembling, they often exhibit complex spatial and kinematic substructures. These infalling regions are vital sites for pre-processing, where localized density enhancements trigger quenching and morphological changes well before the galaxies reach the dense cluster core.
Moreover, as satellite galaxies orbit through the cluster central regions, their interaction between themselves, the Brightest Cluster Galaxy (BCG), and the cluster potential, may strip their outer stellar halo to the point of disruption forming the diffuse intracluster light (ICL).
In this framework, leveraging optical and infrared data, we aim to address the following open questions:
- Pre-processing vs. In Situ Evolution: What fraction of cluster galaxies undergo environmental quenching and morphological transformation via pre-processing within infalling groups, relative to evolution governed by the global cluster halo?
- Progenitors of the Intracluster Light: Which galaxy populations serve as the primary progenitors of the ICL, and to what extent is this diffuse component assembled through the continuous tidal stripping of massive satellites in the core versus the complete disruption of dwarfs in merging substructures?
- Tracing Dynamical Histories: To what extent can the combined mapping of optical substructures and the spatial features of the ICL be used to trace a cluster’s past merger events?
Background characterization, calibration of current satellites and future missions design
The science to be performed on galaxy clusters requires a good knowledge of the response of the instrument (calibration) and a careful treatment of the various background components and of their systematics. It is no joke that «if you want to study the X-ray emission from clusters of galaxies, you will end up actually studying the X-ray background». Our team has been involved in the calibration and background characterization of many X-ray satellites and in particular in recent years, XMM-Newton and NuSTAR. Thanks to this expertise we are very active in the design of new missions such as NewAthena and in the strategy to minimize the level of instrumental background and maximize its knowledge. Based on the physical understanding acquired through the analysis of more than 25 years of XMM data, we have proposed and it is currently under study a Particle Radiation Monitor to fly on board of NewAthena with the necessary accuracy and precision to guarantee the challenging requirement of 2% reproducibility of the background.
Contacts
Silvano Molendi
Fabio Gastaldello
Simona Ghizzardi
Mariachiara Rossetti
Lorenzo Lovisari
Marianna Annunziatella
Beatrice Vaia
Sabrina De Grandi
Former PhD students and Post-Docs
Iacopo Bartalucci
Marco Balboni
Giacomo Riva