Dottorato in ASTRONOMIA

Titolo della tesi: Study of clusters of galaxies by hydrodynamic simulations and observations with cosmological implications

This thesis delves into the critical role of galaxy clusters in constraining the cosmological model through meticulous examinations of cluster observables, primarily, the Sunyaev–Zeldovich (SZ) effect. Accurate cosmological tests based on cluster counts necessitate precise mass estimations, often derived from scaling relations connecting mass proxies to halo mass. This work leverages The Three Hundred(The300) simulations to refine cluster mass estimates and enhance related theoretical understanding in the context of high-angular-resolution observations from the NIKA2 Sunyaev–Zeldovich Large Program (LPSZ). The aim is to understand the systematic effects that cannot be inferred via observations but can affect the estimation of mass (M ) and integrated Compton parameter (Y ) for calibrating a scaling law between the two, with the use of simulations that support real observations. In the first part, a detailed study is performed within the LPSZ, for one of its sample clusters: ACT-CL J0240.0+0116. The multi-wavelength composite reveals a disturbed cluster with non-aligned ICM gas and member galaxy distribution. The SZ inferred pressure profile aligns with the universal model for disturbed clusters, robust to point source contamination treatment. Cluster mass is estimated at 3.84[+1.84,-1.15] × 1014 M⊙ (4.25[+0.50,-0.45] × 1014 M⊙) when modelling (masking) point sources in the pressure profile model, consistent with X-ray and Atacama Cosmology Telescope survey results. A SZ, X-ray combined temperature map reinforces the disturbed state of the cluster. The second part of the thesis uses simulations to validate the LPSZ point source detection algorithm. Through purity and completeness analyses, we establish a framework to quantify the performance of the pipeline and assign reliability scores to detected point sources. The results indicate the best reliability score to be associated with a detection signal of SNR five and the preference for the use of the 260 GHz band for reliable point source detection. These findings are applied to real LPSZ point sources, assigning reliability ranks. The final parts are devoted to theoretical studies by exploiting the The300 simulation products. Twin samples that are representative of the LPSZ are constructed and their NIKA2-like maps that mimic the observations are used to explore the recovery of pressure and mass profiles and calibration of the Y − M scaling law. Discrepancies in the recovered 3D profiles are noted in the cluster core and outskirts. The bias quantifying this discrepancy is minimized at overdensity ∆ = 2000, where the hydrostatic mass bias is 0.15. The hydrostatic mass bias increases to 0.23 at ∆ = 500. The recovered pressure profiles suggest a potential redshift evolution in the universal pressure profile. These pressure and mass estimates, along with varied theoretical estimations of these quantities for multiple projections are used to investigate the Y − M scaling laws at different overdensities while accounting for the projection effect. The projection-induced scatter and the intrinsic scatter of the scaling law are both found to decrease with decreasing ∆. The only exception is the scaling law for mass inferred from the NIKA2-like maps where the intrinsic scatter is minimum at ∆ = 400. The scatter is also lower for cylindrical mass estimates when compared to the spherical ones. The normalisation of the scaling laws complies with expectations and the slope is consistent with a self-similar value of 5/3.