A hydrodynamical view on the mass accretion onto massive halos

This Thesis presents the results of my work on how galaxy clusters form by the accretion of sub-clumps and diffuse materials, and how the accreted energy is distributed in the X-ray emitting plasma. Indeed, on scales larger than tens of millions of light years, the Universe is self-organised by gravity into a spiderweb, the Cosmic Web. Galaxy clusters are the knots of this Cosmic Web, but a strong definition of filaments (which link different knots) and their physical proprieties, is still uncertain. Even if this pattern was determined by studying the spatial distribution of galaxies in the optical band, recently, also in the X-rays probes of filamentary structures around galaxy clusters were obtained. Therefore, given these observational facilities, the galaxy clusters’ outskirts are the best candidate regions to detect filaments and study their physical characteristics. However, from X-rays observations, we have only a few detections of cosmic filaments to date. On the other hand, it is crucial to understand how the accreted energy is dissipated in the baryon content of galaxy clusters and groups. Indeed, it is well known that in the central region of galaxy clusters and groups, the baryon fraction increases with the halo mass. On the outer region, the lack of X-rays constraints influences our understanding of the evolution of baryons in the halos volume. The standard assumption of “closed-box” system, for which the baryon fraction should approach the cosmological ratio Omega_bar/Omega_m, for galaxy clusters and groups seems to be too strong, especially for less massive objects. Moreover, a complete redshift evolution of baryons in galaxy clusters and groups is still missing.

Unveiling the expansion history of the universe with cosmic chronometers and gravitational waves

This Thesis explores two novel and independent cosmological probes, Cosmic Chronometers (CCs) and Gravitational Waves (GWs), to measure the expansion history of the Universe. CCs provide direct and cosmology-independent measurements of the Hubble parameter H(z) up to z∼2. In parallel, GWs provide a direct measurement of the luminosity distance without requiring additional calibration, thus yielding a direct measurement of the Hubble constant H0=H(z=0). This Thesis extends the methodologies of both of these probes to maximize their scientific yield. This is achieved by accounting for the interplay of cosmological and astrophysical parameters to derive them jointly, study possible degeneracies, and eventually minimize potential systematic effects. As a legacy value, this work also provides interesting insights into galaxy evolution and compact binary population properties. The first part presents a detailed study of intermediate-redshift passive galaxies as CCs, with a focus on the selection process and the study of their stellar population properties using specific spectral features. From their differential aging, we derive a new measurement of the Hubble parameter H(z) and thoroughly assess potential systematics. In the second part, we develop a novel methodology and pipeline to obtain joint cosmological and astrophysical population constraints using GWs in combination with galaxy catalogs. This is applied to GW170817 to obtain a measurement of H0. We then perform realistic forecasts to predict joint cosmological and astrophysical constraints from black hole binary mergers for upcoming gravitational wave observatories and galaxy surveys. Using these two probes we provide an independent reconstruction of H(z) with direct measurements of H0 from GWs and H(z) up to z∼2 from CCs and demonstrate that they can be powerful independent probes to unveil the expansion history of the Universe.

Galaxy cluster cosmology in KiDS, Planck, and Euclid surveys

In this Thesis, we present a series of works that encompass the fundamental steps of cosmological analyses based on galaxy clusters, spanning from mass calibration to deriving cosmological constraints through counts and clustering. Firstly, we focus on the 3D two-point correlation function (2PCF) of the galaxy cluster sample by Planck Collaboration XXVII (2016). The masses of these clusters are expected to be underestimated, as they are derived from a scaling relation calibrated through X-ray observations. We derived a mass bias which disagrees with simulation predictions, consistent with what derived by Planck Collaboration VI (2020). Furthermore, in this Thesis we analyse the cluster counts and 2PCF, respectively, of the photometric galaxy cluster sample developed by Maturi et al. (2019), based on the third data release of KiDS (KiDS-DR3, de Jong et al. 2017). We derived constraints on fundamental cosmological parameters which are consistent and competitive, in terms of uncertainties, with other state-of-the-art cosmological analyses. Then, we introduce a novel approach to establish galaxy colour-redshift relations for cluster weak-lensing analyses, regardless of the specific photometric bands in use. This method optimises the selection completeness of cluster background galaxies while maintaining a defined purity threshold. Based on the galaxy sample by Bisigello et al. (2020), we calibrated two colour selections, one relying on the ground-based griz bands, and the other including the griz and Euclid YJH bands. In addition, we present the preliminary work on the weak-lensing mass calibration of the clusters detected by Maturi et al. (in prep.) in the fourth data release of KiDS (KiDS-1000, Kuijken et al. 2019). This mass calibration will enable the cosmological analyses based on cluster counts and clustering, from which we expect remarkable improvements in the results compared to those derived in KiDS-DR3.

A comprehensive study of the AGN feedback cycle in galaxy clusters from high resolution X-ray and radio observations

At the center of galaxy clusters, a dramatic interplay known as feedback cycle occurs between the hot intracluster medium (ICM) and the active galactic nucleus (AGN) of the central galaxy. The footprints of this interplay are evident from X-ray observations of the ICM, where X-ray cavities and shock fronts are associated with radio lobe emission tracing energetic AGN outbursts. While such jet activity reduces the efficiency of the hot gas to cool to lower temperatures, residual cooling can generate warm and cold gas clouds around the central galaxy. The condensed gas parcels can ultimately reach the core of the galaxy and be accreted by the AGN. This picture is the result of tremendous advances over the last three decades. Yet, a deeper understanding of the details of how the heating–cooling regulation is achieved and maintained is still missing. In this Thesis, we delve into key aspects of the feedback cycle. To this end, we leverage high-resolution (sub-arcsecond), multifrequency observations (mainly X-ray and radio) of several top-level facilities (e.g., Chandra, JVLA, VLBA, LOFAR). First, we investigate which conditions trigger a feedback response to gas cooling, by studying the properties of clusters where feedback is just about to start. Then, we focus on the details of how the AGN–ICM interaction progresses by examining cavity and shock heating in the cluster RBS797, an exemplary case of the jet feedback paradigm. Furthermore, we explore the importance of shock heating and the coupling of distinct jet power regimes (i.e., FRII, FRI and FR0 radio galaxies) to the environment. Ultimately, as heating models rely on the connection between the direct evidence (the jets) and the smoking gun (the X-ray cavities) of feedback, we examine the cases in which these two are dramatically misaligned.

Amplification mechanisms of inflationary spectra in induced gravity

Inflation is the primordial stage of accelerated expansion of the Universe which solves the issues of the initial conditions of a decelerating Universe (horizon, flatness and entropy problems). Moreover, it is supposed that quantum fluctuations originated during the first moments after the Big Bang gave rise to the formation of galaxies and other structures of the Universe when inflation ends. Among these structures also primordial black holes (PBHs) may have been generated. The interest in PBHs relies on their possible connection with dark matter: they could constitute a portion or even the whole dark matter content of our Universe.\\ In this work we consider inflation in the Induced Gravity (IR) context and study possible mechanisms of amplification of the curvature perturbations generated during the cosmic acceleration. In particular we consider the possibility of a period of Constant Roll (CR). Starting from the previous work of Starobinsky et al. Our final purpose is to analyse the power spectrum of the scalar perturbations and to find in which conditions there is an enhancement of the power spectrum possibly leading to PBHs formation.

Gravitational microscope in MACSJ1206: unveiling galaxy cluster member mass distribution

Gravitational lensing is a powerful tool to investigate the properties of the distribution of matter, be it barionic or dark. In this work we take advantage of Strong Gravitational Lensing to infer the properties of one of the galaxy-scale substructures that makes up the cluster MACSJ1206. It is relatively easy to model the morphology of the visible components of a galaxy, while the morphology of the dark matter distribution cannot be so easily constrained. Being sensitive to the whole mass, strong lensing provides a way to probe DM distribution, and this is the reason why it is the best tool to study the substructure. The goal of this work consists of performing an analysis of the substructure previously mentioned, an early type galaxy (ETG), by analyzing the highly magnified Einstein ring around it, in order to put stringent constraints on its matter distribution, that, for an ETG, is commonly well described by an isothermal profilele. This turns out to be interesting for three main different reasons. It is well known that galaxies in clusters are subject to interaction processes, both dynamic and hydrodynamic, that can significantly modify the distribution of matter within them. Therefore, finding a different profile from the one usually expected could be a sign that the galaxy has undergone processes that have changed its structure. Studying the mass distribution also means studying the dark matter component, which not only still presents great questions today, but which is also not obviously distributed in the same way as in an isolated galaxy. What emerges from the analysis is that the total mass distribution of the galaxy under examination turns out to have a slope much steeper than the isothermal usually expected.

Investigating stellar mixing processes through the study of the Red-Giant Branch bump

Extra mixing at the borders of convective zones in stellar interiors takes on an important role in the chemical evolution of stars and galaxies through the transport of chemical elements towards the stellar surface: knowing the overshooting mechanism can therefore lead to a better understanding of the observed chemical abundances in stellar photospheres. The comprehension of this phenomenon is quite uncertain and currently object of many studies. In particular, concerning low mass stars, in the past decades several works highlighted a discrepancy between the observed luminosity of the Red-Giant Branch bump and its prediction from simulations, which can be fixed including overshooting at the base of the convective envelope. This work, studying the Red-Giant Branch bump and using it as a diagnostic for extra mixing processes, tries to classify two different types of overshooting, instantaneous and diffusive, using both simulations from stellar models and Globular Clusters’ data. The aim is to understand which one of the two mixing processes is the most suitable in reproducing the observed stellar behaviour and, in case both of them provide reliable results, what are the conditions under which they produce the same effects on the Red-Giant Branch bump luminosity function and are consequently indistinguishable. Finally, possible dependences of overshooting efficiency on stellar parameters, such as chemical composition, are analysed.

Constraints on the duty cycle of the remnant radio galaxy NGC 6086 in the galaxy group Abell 2162

Galaxy clusters and groups are the most massive bounded structures and the knots of the large-scale structure of the Universe. These structures reside in dark matter haloes, hosting tens to hundreds of galaxies and they are filled with hot and rarefied gas. Radio Galaxies are a peculiar class of galaxies with a luminosity in the radio band up to 10^46 erg/s between 10 MHz and 100 GHz. These galaxies are a subclass of AGN in which there is accretion on the Super Massive Black Hole. The accretion generates jets of relativistic particles and magnetic fields which lose energy through synchrotron radiation, best observable at radio frequencies. The study of the spectral ageing of the AGN plasma is fundamental to understand its evolution, interaction with the environment and to constrain the AGN duty cycle. n this thesis, we have investigated the duty cycle of the nearby remnant radio galaxy NGC 6086, located in the centre of the galaxy group Abell 2162. We have made major steps forward thanks to the new high-sensitivity interferometers in the low-frequency radio band. We have detected for the first time three filaments of emission and a second couple of lobes. We have performed an integrated and resolved analysis on the previously known inner lobes, the new filaments and the older outer lobes. We have performed an age estimate of the two pairs of lobes to give constraints on the duty cycle of the source and an estimate of its active time.

Stellar mass loss from the Fornax dwarf spheroidal galaxy

Dwarf galaxies often experience gravitational interactions from more massive companions. These interactions can deform galaxies, turn star formation on or off, or give rise to mass loss phenomena. In this thesis work we propose to study, through N-body simulations, the stellar mass loss suffered by the dwarf spheroid galaxy (dSph) Fornax orbiting in the Milky Way gravitational potential. Which is a key phenomenon to explain the mass budget problem: the Fornax globular clusters together have a stellar mass comparable to that of Fornax itself. If we look at the stellar populations which they are made of and we apply the scenarios of stellar population formation we find that, originally, they must have been >= 5 times more massive. For this reason, they must have lost or ejected stars through dynamic interactions. However, as presented in Larsen et al (2012), field stars alone are not sufficient to explain this scenario. We may assume that some of those stars fell into Fornax, and later were stripped by Milky Way. In order to study this solution we built several illustrative single component simulations, with a tabulated density model using the P07ecc orbit studied from Battaglia et al (2015). To divide the single component into stellar and dark matter components we have defined a posterior the probability function P(E), where E is the initial energy distribution of the particles. By associating each particle with a fraction of stellar mass and dark matter. In this way we built stellar density profiles without repeating simulations. We applied the method to Fornax using the profile density tables obtained in Pascale et al (2018) as observational constraints and to build the model. The results confirm the results previously obtained with less flexible models by Battaglia et al (2015). They show a stellar mass loss < 4% within 1.6 kpc and negligible within 3 kpc, too small to solve the mass budget problem.