Mass and selection biases of galaxy clusters: a multi-probe approach

Galaxy clusters are fascinating astrophysical objects with a great interest for cosmological applications.

The recipe: Baryons, detectable at different wavelengths, represent only small amount in the mass budget (12% in the diffuse IntraCluster Medium, ICM, and 3% confined in galaxies), while Dark Matter, DM, is the dominant ingredient (85%) but unfortunately only traceable through its gravitational impact. 

Up to now, clusters have been mostly selected by their minority component: directly through the emission from galaxies or the hot ICM, or through the scattering of the Cosmic Microwave Background (CMB) on the ICM (Sunayev-Zeldovich, SZ, effect). These selection criteria are convenient, have provided us with large samples at various redshifts and have helped us establish many of the cluster properties. However, the samples are biased and the amplitude of the bias is yet to be determined. 

Furthermore, knowledge of the cluster mass is needed for both cosmological and astrophysical studies, but mass cannot be directly observed and needs to be derived from the collected photons. 

Our understanding of galaxy cluster physics, and our ability to use clusters to constrain cosmological parameters, are limited by mass and selection biases of the studied samples. 

Given the critical role played by the mass and selection biases, in this project we (Unit 2) exploit the only sample available so far that contains a sizable fraction of low surface brightness clusters missed in X-ray- and SZ-selected surveys. We aim at better determining the role of the selection bias (against low surface brightness clusters) in scaling relations of cosmological and astrophysical interest with the assistance of our data and hydrodynamic simulations; and we (Unit 2) investigate the amount of mass bias by using multi-probe approaches based on high-resolution and high-sensitivity observations with the support of state-of-the-art hydrodynamical simulations.

With this project, we can address, uniquely and for the first time, the mass bias of a sample non-ICM biased and the impact of the ICM selection on the mass bias by comparing ICM-biased and ICM-unbiased samples. We hope to overcome the limitation of literature studies that address mass bias and selection effects independently.

Research Unit 2 - Personnel of Sapienza University unit

Marco De Petris (Associate Professor) Co-I

Antonio Ferragamo (postdoc)

Luca Lamagna (Associate Professor)

Raphael Wicker (postdoc)

Aishwarya Paliwal (PhD student)

Sara Santoni (PhD student)

+ Master students: Giuseppe Giulio Baris, Alessio Benincasa, Francesco Guidi and Elia Pappalardo. 

Research Unit 2 - Projects related on Mass inference and possible biases

• Almost all the research activities related to the PRIN 2022 project were carried out within the framework of the following collaborations: 

The300, hydrodynamical simulation

NIKA2, observations of 38 clusters at 150 and 260 GHz from IRAM 30-m telescope at Pico Veleta (Large Program Sunayev-Zeldovich)

CHEX-MATE, (Cluster HEritage project with XMM-Newton) observations of 118 clusters with XMM-Newton satellite at X-ray, and 

EURA NOVA, data science company.

• Cluster mass inference could be impacted by cluster dynamical state. Studying the most efficient morphological indicators, or a combination of them, to infer the dynamical state with survey-quality data and with hydrodynamical simulations is mandatory. We proposed a novel and promising approach to modeling cluster morphology using Zernike polynomials (ZPs). This parameter, already validated on SZ maps generated from synthetic clusters in The300, was recently extended to images with different angular resolutions and wavelengths, and subsequently to real data, to infer the dynamical state. More specifically in: 

  1. 1. 1. Inference of morphology and dynamical state of nearby Planck-SZ galaxy clusters with Zernike polynomials, Capalbo, De Petris et al. 2025;

        2. NIKA2 LPSZ Results VII: Clusters dynamical state inference by Zernike polynomials on high-angular resolution tSZ images, Pappalardo et al. (De Petris). (to be submitted to A&A in 2026 together with the first NIKA2 LPSZ results papers);

        3. CHEX-MATE: Classification of galaxy clusters dynamical state with Zernike Polynomials on X-ray images, Ferragamo et al. (De Petris) (under revision CHEX-MATE collaboration, to be submitted to A&A (2026));

        4. optical images (galaxy number density maps), Ferragamo, De Petris et al. analysis underway .

Other different metrics can be used to classify cluster dynamical state: a possibility has been the recent use of two different statistical approaches – Principal Component Analysis (PCA) and Uniform Manifold Approximation and Projection (UMAP), as we explored in Haggar, De Luca, De Petris et al. 2024.

• Inference of a minimally biased total mass by applying Machine Learning techniques. This activity is based on a strong/old collaboration with the Universidad Autonoma de Madrid (G. Yepes, W. Cui, D. de Andres and collabs) and EURA NOVA, a company working on big data. We started by inferring the total mass of Planck clusters, i.e., without any mass bias, by training a CNN on The300 Compton-y maps and then applying it to real Planck maps. We then explored the reconstruction of unbiased 3D total (and gas) mass radial profiles starting from mock Compton-y maps. Within the PRIN 2022, we carried out the following activities: 

  1. 1. 1. Deep learning generated observations of galaxy clusters from dark-matter-only simulations, Caro et al. (De Petris) 2025;

        2. Reconstruction of total mass radial profiles of NIKA2 Twin Samples by ML model, Ferragamo, Baris, De Petris et al. analysis underway;

        3. The impact of unbiased Planck clusters masses, inferred by CNN model, impact cosmological parameters, Wicker, De Petris, et al. analysis underway.

• Impact of filaments on cluster properties, such as mass and bias. How much the filamentary structures around clusters are correlated with their properties? 

  1. 1. 1. Cluster connectivity, estimated by gas density field, is recostructured in The300 regions at z=0 using the filament finder DisPerSE and related to their mass, Santoni, De Petris et al. 2024.

        2. The peculiar case of short filaments, i.e. bridges, studied on Atacama Cosmology Telescope maps with the support of The300 mock images, Isopi et al. (De Petris) 2025

        3. Cosmic Web extraction from 2D gas and Compton-y maps of clusters outskirts to study the impact of line of sight projection on filament reconstruction using The300, Santoni, De Petris et al. (in preparation).

        4. Mapping intracluster gas filaments in the X-ray maps of CHEX-MATE, Santoni S., I. Bartalucci, M. De Petris, et al. analysis underway.

• To better understand the thermodynamics and spatial distribution of the ICM in clusters, and thus potentially obtain more accurate cluster mass estimates, we take advantage of the high angular resolution and sensitivity of NIKA2 tSZ observations. We had the opportunity to contribute to and lead several projects together with the NIKA2 LPSZ team. Some results have already been published, while others are still pending and will be submitted together with the main first NIKA2 LPSZ results papers, with submission to A&A planned for early 2026. In the following the main analyses related to the PRIN activity.

  1. 1. 1. The mapping of the ICM and the inference of the dynamical state of ACT-CL J0240.0+0116 using a multi-wavelength approach are reported in Paliwal, De Petris et al. 2025.

        2. The capability of double band, high resolution observations to blindly detect clusters in the NIKA2 survey, Chérouvrier et al. (De Petris) 2025 

        3. All the following projects, related to NIKA2 LPSZ observations, will appear early 2026 as NIKA2 LPSZ Results papers, at the moment submitted to A&A. 

           • all the high-angular resolution maps of 35 clusters of the galaxy cluster sample of the Sunyaev-Zeldovich NIKA2 Large Programme with relevant data, Hanser et al. (De Petris) 2026a;

           • Point-source population across LPSZ clusters of galaxies shoeing an evidence for a gravitational lensing effect, Desert et al. (De Petris) 2026;

           • Thermodynamic profiles of this sample of medium/high-redshift clusters (0.5<z<0.9) from high angular resolution SZ and X-ray synergy, Hanser et al. (De Petris) 2026b;

           • The scaling relation between SZ observables and the hydrostatic equilibrium masses, Moyer-Anin et al. (De Petris) 2026;

           • Turbulence in the intracluster medium as seen through Sunyaev-Zel’dovich fluctuations, Adam et al. (De Petris) 2026;

        4. The NIKA2 high resolution also allows to map shocks in disturbed high redshift cluster, such as PSZ2G091.83+26.11 and to show their impact on mass estimates, Artis et al. (De Petris), to be submitted to A&A.

        5. Gas pressure profiles, reconstructed from the NIKA2 Twin Sample mock images at 150 GHz provided by The300 simulations, are compared with real NIKA2 profiles and other profiles available in literature, Paliwal, De Petris, et al., analysis underway.

• The Sparsity in clusters, a common parameter based on the ratio of spherical halo masses estimated at two radii enclosing different overdensities, is useful also to infer constraints on cosmological model parameters. However, it is important to account for the impact of possible mass biases, such as the hydrostatic mass bias. This has been investigated using The300 data in Corasaniti et al. (De Petris) 2025.

• For very distant objects, where X-ray spectrometers are starved of photons, it is difficult to obtain an accurate map of the temperature of the gas in the clusters. This could affect the estimate of the hydrostatic mass. The combination of SZ and X-ray intensity observations allows to reconstruct such kind of maps.

  1. 1. 1. We have validated a model to recover mass-weighted temperature maps using simulated SZ and X-ray images of The300 clusters including the observational impacts of NIKA2 and XMM-Newton instruments, Wicker, De Petris et al., submitted to A&A (2025). 

        2. The application on real NIKA2 LPSZ with XMM-Newton maps is on going, Wicker, De Petris et al., analysis underway. 

• Thanks to the XMM-Newton observations from the CHEX-MATE project, it has been possible to investigate ICM profiles in a large sample of clusters, including both nearby low- to intermediate-mass systems and massive distant ones.

  1. 1. 1.  A jointly fit of the universal pressure profile and individual cluster M500 masses over a sample of 25 galaxy clusters of CHEX-MATE has demonstrated to reduce biases, Muñoz-Echeverría et al. (De Petris) 2025; 

        2. the impact cluster's triaxiality and orientation on Planck SZ cluster selection and weak lensing mass measurements, Saxena et al. (De Petris) 2025;

        3. to assess the reliability of current analysis techniques in reconstructing key thermodynamic properties , with particular focus on samples similar to those observed in the CHEX-MATE, Seppi et al. (De Petris) submitted to A&A (2025).

• Finally, the baryon content in galaxy clusters is investigated using The300 simulations dataset to assess the  possible impact of sample selection on the derived cosmological parameters. The evolution of baryons and, more specifically, the hot gas is reported in Rasia et al. (De Petris) 2025. With almost the same dataset, we studied the peculiar population of clusters with low gas content: their environment and impact on Y-M scaling law, F. Guidi, M. De Petris, et al. analysis underway .

Research Unit 2

Publications funded by PRIN 2022   

1. V. Capalbo, M. De Petris, A. Ferragamo, et al. "Inference of morphology and dynamical state of nearby Planck-SZ galaxy clusters with Zernike polynomials" A&A, 698, A201 (2025),  https://doi.org/10.1051/0004-6361/202452649

2. A. Caro, D. de Andres, W. Cui, et al + M. De Petris, "Deep learning generated obser v ations of galaxy clusters from dark-matter-only simulations ", RAS Techniques and Instruments, Volume 4, (2025), rzaf007, https://doi.org/10.1093/rasti/rzaf007

3. D. Chérouvrier, et al. + M. De Petris, "NIKA2 Cosmological Legacy Survey - Blind detection of galaxy clusters in the COSMOS field via the SZ effect", A&A, 700, A30 (2025), https://doi.org/10.1051/0004-6361/202554450

4. P.S. Corasaniti, et al. + M. De Petris, "The impact of baryons on the sparsity of simulated galaxy clusters from The Three Hundred Project: Astrophysical and cosmological implications", A&A, 697, A33 (2025) https://doi.org/10.1051/0004-6361/202553914

5. R. Haggar, F. De Luca, M. De Petris, et al. "Reconsidering the dynamical states of galaxy clusters using PCA and UMAP" MNRAS, 532, 1, (2024),  https://doi.org/10.1093/mnras/stae1566 

6. G. Isopi, et al. + M. De Petris, "The Atacama Cosmology Telescope: A census of bridges between galaxy clusters", JCAP 08 (2025) 078, doi:10.1088/1475-7516/2025/08/078 

7. M. Muñoz-Echeverría, et al. + M. De Petris, "CHEX-MATE: towards a consistent universal pressure profile and  cluster mass reconstruction", A&A, 704, A302 (2025) https://doi.org/10.1051/0004-6361/202556045

8. A. Paliwal, M. De Petris, A. Ferragamo, et al. "Exploiting the high-resolution NIKA2 data to study the intracluster medium and dynamical state of ACT-CL J0240.0+0116" A&A, 698, A2 (2025),  https://doi.org/10.1051/0004-6361/202452597

9. E. Rasia, et al. + M. De Petris, "The Three Hundred Project: Modeling baryon and hot-gas fraction evolution in simulated clusters", A&A, 702, A182 (2025), https://doi.org/10.1051/0004-6361/202554283

10. S. Santoni, M. De Petris, G. Yepes, et al. "The Three Hundred project: Estimating the dependence of gas f ilaments on the mass of galaxy clusters " A&A, 692, A44 (2024), https://doi.org/10.1051/0004-6361/202450895 

11. H. Saxena, et al. + M. De Petris, "CHEX-MATE: The impact of triaxiality and orientation on Planck SZ cluster selection and weak lensing mass measurements" A&A, 700, A128 (2025) https://doi.org/10.1051/0004-6361/202555719 

in preparation, submitted and under review 

1. R. Adam, et al. + M. De Petris, "NIKA2 LPSZ Results XI: Turbulence in the intracluster medium as seen through Sunyaev-Zel’dovich fluctuations", submitted to A&A (2026)

2. E. Artis, et al. + M. De Petris, "NIKA2 LPSZ: High resolution study of the complex massive and high redshift cluster PSZ2G091.83+26.11", to be submitted to A&A (2026)

3. F.X. Desert, et al. + M. De Petris, "NIKA2 LPSZ Results II: Point-source population across clusters of galaxies Evidence for a gravitational lensing effect", submitted to A&A (2026)

4. A. Ferragamo, A. Benincasa, M. De Petris, et al "CHEX-MATE: Classification of galaxy clusters dynamical state with Zernike Polynomials on X-ray images" under revision CHEX-MATE collaboration, to be submitted to A&A (2026)

5. A. Ferragamo, G. Baris, M. De Petris, et al. "NIKA2 LPSZ Results XV. Mass estimates from tSZ using Machine Learning model" in preparation (2026)

6. C. Hanser (a), L. Perotto, et al + M. De Petris, "NIKA2 LPSZ Results I. High-angular resolution maps of the galaxy cluster sample of the Sunyaev-Zeldovich NIKA2 Large Programme", submitted to A&A (2026)

7. C. Hanser (b), I. Bartalucci, et al + M. De Petris, "NIKA2 LPSZ Results III. Thermodynamic profiles of a sample of high-redshift clusters from high angular resolution SZ and X-ray synergy", to be submitted to A&A (2026)

8. A. Moyer-Anin, F. Kéruzoré, et al. + M. De Petris, "NIKA2 LPSZ Results V. The scaling relation between SZ observables and the hydrostatic equilibrium masses", to be submitted to A&A (2026)

9. E. Pappalardo, A. Ferragamo, et al. + M. De Petris "NIKA2 LPSZ Results VII: Clusters dynamical state inference by Zernike polynomials on high-angular resolution tSZ images", to be submitted to A&A (2026)

10. S. Santoni, M. De Petris, et al. "The Three Hundred project: Cosmic Web extraction from 2D gas and Compton-y maps of clusters outskirts", in preparation (2026)

11. S. Santoni, I. Bartalucci, M. De Petris, et al. "Study of gas filamentary structures within CHEX-MATE galaxy clusters", in preparation (2026)

12. R. Seppi, et al. + M. De Petris, "CHEX-MATE: Are we getting cluster thermodynamics right?", submitted to A&A (2025) https://arxiv.org/pdf/2603.03440

13. R. Wicker, M. De Petris, et al., "Galaxy cluster temperature maps from joint X-ray and tSZ maps with The Three Hundred hydrodynamical simulations", submitted to A&A (2025) https://arxiv.org/pdf/2511.08183

14. R. Wicker, M. De Petris, et al., "NIKA2 LPSZ Results IX: SZ and X-ray temperature maps", in preparation (2026)