Our Research & Team

Welcome to the CLEVeR (CLuster and group Environment as Viewed by eROSITA) research project! Here, we delve into the mysteries of the universe, exploring clusters and groups of galaxies as viewed by eROSITA. Our mission is to unravel the secrets of large-scale structure, and galaxy formation and evolution and share our discoveries with the world.

At CLEVeR, we are dedicated to pushing the boundaries of astrophysical research. Our team of passionate scientists and researchers work tirelessly to uncover the hidden truths of the cosmos. We strive to engage not only the scientific community but also the wider audience, sparking curiosity and fascination for the wonders of the universe.

Galaxy Groups: The Missing Link in Cosmic Evolution

Galaxy groups are the hidden key to unlocking the mysteries of galaxy formation and large-scale structure. These systems are where most galaxies spend their lives, yet they remain one of the biggest enigmas in modern cosmology. The central player? The supermassive black hole (SMBH) at the heart of the group’s dominant galaxy.

At this scale, AGN feedback is a game-changer—it can heat, expel, or redistribute gas, determining whether galaxies flourish or fade. However, this influence can make galaxy groups harder to detect in X-rays, leading to a "streetlight effect"—where we only see the brightest, most gas-rich systems while missing those affected by AGN-driven gas removal.

To truly measure how AGN feedback shapes galaxy groups, we can’t just look where X-rays are brightest—we must also look where X-ray emission is weak or absent. CLEVeR tackles this challenge by combining eROSITA’s X-ray vision with optical surveys to identify hidden and faint galaxy groups, revealing how SMBHs impact gas content and star formation. By stacking optically selected galaxy groups, CLEVeR provides a complete picture of AGN feedback’s role in galaxy evolution and cosmic structure formation.

Simulating the X-ray Universe

To accurately test how feedback influences the X-ray appearance of galaxy groups, CLEVeR employs an innovative approach: synthetic eROSITA observations derived from cosmological simulations. Theoretical models of AGN feedback predict widely different outcomes, from gas-rich, X-ray-bright halos to systems completely stripped of their hot gas. To bridge this gap between models and observations, we need a robust theoretical-to-observational comparison framework.

How We Use Light-Cones to Test Feedback Effects

CLEVeR leverages high-resolution hydrodynamical simulations such as IllustrisTNG, Magneticum, and BahamasXL to create realistic light-cone projections of galaxy groups across cosmic time. These light-cones capture the 3D structure and evolution of baryons, enabling us to:

Trace the impact of feedback on gas expulsion and heating over billions of years.
Reconstruct how the circumgalactic and intra-group medium evolves under different AGN feedback scenarios.
Compare simulated X-ray observations with real eROSITA data, accounting for instrumental biases and selection effects.

By converting these light-cone simulations into synthetic eROSITA event files, we can apply the same data analysis techniques used for real observations. This ensures a one-to-one comparison between theoretical predictions and observed scaling relations, allowing us to validate or refine feedback implementations in galaxy evolution models.

Real eROSITA observation of the eFEDS (The eROSITA Final Equatorial-Depth Survey, Ghirardini et al. 2022)

Mock eROSITA observation derived from Magneticum (Marini et al. 2024)

The Link Between Central Galaxy Properties and CGM Conditions

The circumgalactic medium (CGM) acts as both a fuel reservoir for star formation and a channel for AGN feedback, yet its properties vary significantly between galaxies of similar mass. Why do some galaxies retain cool, star-forming gas while others host a hot, depleted CGM?

Observations show that star-forming galaxies are surrounded by dense, multiphase CGM, while quiescent galaxies have hot, structureless halos. This suggests a strong link between gas cooling, AGN activity, and galaxy evolution.

CLEVeR will investigate this connection using eROSITA’s X-ray data, combined with optical and radio surveys to:

Map X-ray luminosity profiles to determine where AGN feedback prevents cooling.
Test the precipitation framework, exploring whether feedback-triggered uplift fuels or suppresses star formation.
Correlate CGM conditions with AGN activity, using radio jets (LOFAR), emission lines (BPT diagnostics), and star formation rates.

By linking central galaxy properties to CGM thermodynamics, CLEVeR will provide direct observational tests of AGN-driven self-regulation, reshaping our understanding of galaxy and halo co-evolution

Effect of different AGN feedback implementation on the Circum Galactic Medium and the halo gas in galaxy groups (left panels) and a massive galaxy clusters (right panels) from different simulations. The upper panels show the predicted hot gas density distributions, while the lower panels show the temperature distrubution (Pillepich et al. 2017).

Explore Our Publications

Unraveling the Quenching of Satellite Galaxies

Satellite galaxies follow a unique evolutionary path, shaped by environmental processes that gradually suppress their star formation. As they move through their host halos, they undergo strangulation (losing their gas reservoirs), ram-pressure stripping (gas removal by the surrounding medium), tidal stripping (gravitational interactions), and harassment (repeated encounters with other galaxies).

CLEVeR leverages eROSITA’s X-ray data alongside optical, radio, and spectroscopic surveys to:

Identify thousands of satellite galaxies and trace their gas depletion.
Determine how different quenching mechanisms operate across environments.
Test the “delayed-then-rapid” quenching scenario, where satellites form stars for a few gigayears before sudden gas loss.

By mapping how satellites lose their fuel for star formation, CLEVeR provides a key observational test for galaxy evolution models, revealing the fate of galaxies in groups and clusters.

The various quenching pathways and what happens to the cold gas component of the interstellar medium (blue). Each sequence begins with the galaxy no longer accreting gas from the surrounding area (pink). The colour of the stars (from young, blue stars to old, red stars) indicates the stage of quenching. Credit: Cortese et al. (2021)