Ingoglia, Lorenzo (2022) Cosmological probes from weak lensing analysis on galaxy clusters. [Tesi di dottorato]

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The Universe is composed of matter distributed on the large scale as a web-like structure. Galaxy clusters are located at the densest regions of the cosmic structure, the dark matter halos. Therefore, studying this class of objects gives crucial insights into the evolution of the matter distribution in the Universe and precious clues on the nature of the enigmatic dark matter. The dark matter, largely dominant in the halos, is invisible with direct observations. However, we can determine the dark matter distribution around galaxy clusters by means of weak gravitational lensing. This method takes advantage of the deflection of light induced by massive objects, namely clusters of galaxies, to derive their mass density profiles on scales reaching the halo boundaries and beyond, extending into the regime of the large-scale structure. Galaxy clusters and their hosts, the halos, are biased tracers of the underlying matter density field. This effect is characterized by the so-called halo bias, a parameter scaling the density profiles driven by the correlated matter distribution around galaxy clusters. During my thesis, I investigated the relation between the halo mass and the halo bias derived from stacked weak lensing profiles of about 7000 AMICO galaxy clusters. This catalog is assessed from the third data release of KiDS, the ESO public survey. Stacking the profiles is a process that reduces the statistical noise of the lensing signal and increases the quality of the measured parameters. We thus split the cluster sample into 14 redshift-richness bins and derived the halo bias and the virial mass in each bin by means of a standard Bayesian inference. It is carried out by a fiducial density model broken in a one-halo term, identified with the galaxy cluster halo and its physical characteristics (mass, concentration, etc), and a two-halo term, associated with matter distributed in distinct pairs of halos and directly proportional to the halo bias. The two terms of the halo profile correlate in such a way that the halo bias follows an increasing function of mass. This relation has been shown and modeled in several theoretical studies based on \textit{N}-body numerical simulations, in the framework of the $\Lambda$CDM standard cosmological model. The results of our study show an agreement within $2\sigma$ between our estimation of the halo bias and theoretical predictions. The measurements of the average mass and bias over the stacked density profile of the full cluster catalog give $M_{200c} = (4.9 \pm 0.3) \times 10^{13} M_{\odot}/\textit{h}$ and $b_h \sigma_8^2 = 1.2 \pm 0.1$. Considering the degenerated form of the halo bias and the additional prior of a bias-mass relation from numerical simulations, we constrained the normalization of the matter power spectrum. We found $\sigma_8 = 0.63 \pm 0.10$ with the matter density of the Universe set at $\Omega_m = 0.3$. Even if a fixed cosmology does not allow to complete a fully independent cosmological inference, this result agrees with other studies based on CMB data, cluster clustering, cluster counts, and cosmic shear analyses within $2\sigma$. In the upcoming years, the next generation of sky surveys will provide deeper and wider catalogs of data for cosmologists to answer modern inquiries. As part of my thesis, I have been involved in the development of a numerical tool in the context of the Science Ground Segment data processing pipeline of the Euclid consortium. COMB-CL is a python module (at the moment, in a state of prototype) that aims to measure the weak lensing mass of galaxy clusters. The code is built in such a way that catalogs of cluster and galaxy properties (position, redshift, shear, color, etc) are input and, given a fiducial cosmology and a model for the halo density profile, catalogs of weak lensing profiles and related masses are output. This toolkit has been accepted and will be reviewed in a paper of the key project LE3-CL-2 regarding the characterization of the properties of detected galaxy clusters.

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