Quasar clustering measurements provide a powerful probe of the physical processes that govern the growth of supermassive black holes and the coevolution with their host galaxies. Observations from wide-field quasar surveys indicate that the quasar auto-correlation length increases dramatically from z~2 to z~4. The clustering amplitude measured at z~4 is so extreme that it has been difficult to reproduce theoretically, even when assuming that quasars are hosted by the most massive dark matter halos residing in the most extreme environments at that redshift. As the first project of my Ph.D., I focused on this decade-long problem and revisited it thanks to the new, large-volume, N-body cosmological simulations that have been developed in recent years by multiple teams. I created a model that simultaneously reproduces both the observed quasar clustering and quasar demographics. The model builds on a novel method to compute the halo mass and halo cross-correlation functions by combining multiple large-volume dark-matter-only cosmological simulations with different box sizes and resolutions. Armed with these halo properties, the model exploits the conditional luminosity function framework to describe the stochastic relationship between quasar luminosity, L, and halo mass, M, and it is able to quickly predict observables such as the quasar auto-correlation function and the quasar luminosity function as well as other relevant quantities such as the quasar luminosity-halo mass relation, the quasar host mass function, and the quasar duty cycle. We applied this model to observations of the quasar clustering and demographics at z~2 and z~4, and found that we can reproduce all the data very well. The very large clustering amplitude at z~4 implies that the relation between quasar luminosity and halo mass is very steep and has very little intrinsic scatter. This has profound consequences in terms of accretion physics and the growth of supermassive black holes at high redshift. (figure from Pizzati et al. 2024a)

Since its launch at the end of 2021, the James Webb Space Telescope (JWST) is rapidly revolutionizing our understanding of the high-redshift Universe. One of the most powerful JWST instrument for studying the early Universe is the NIRCam Wide Field Slitless Spectroscopy mode, which allows for the spectroscopic identification of high-redshift quasars and galaxies over large comoving volumes. Exploiting NIRCam/WFSS observations of several bright high-z quasars fields, the EIGER survey has identified many [OIII]-emitting galaxies that are physically associated with the targeted quasars, revealing a variety of large-scale enviroments around the quasars, included one of the most spectacular overdensities revealed in the Epoch of Reionization. By putting together different quasar fields, in Eilers et al. 2024 we have measured for the first time the quasar-galaxy cross-correlation function at z~6, together with the galaxy auto-correlation function at the same redshift. These measurements hold the key to understanding the properties of quasars and galaxies at high redshift and their relation with the host dark matter halos. However, the interpretation of these data is not straightforward, and requires the development of a theoretical framework that can simultaneously reproduce the observed properties of quasars and galaxies at z~6. By building upon the empirical quasar population model described above (see Pizzati et al. 2024a), we have worked on developing such framework. In this context, we made use of a new, large-volume N-body simulation with more than a trillion particles, FLAMINGO-10k, that is able to model quasars and galaxies simultaneously. We successfully reproduced observations of z~6 quasars and galaxies (i.e., their clustering properties and luminosity functions), and used them to infer key quantities such as their luminosity-halo mass relation, the mass function of their host halos, and their duty cycle/occupation fraction. By focusing on the inferred properties of quasars, we presented for the first time a homogeneous analysis of their evolution up to z>6. The picture that emerges reveals a strong evolution of the host halo mass and duty cycle of quasars at z~2-6, and calls for new investigations of the role of quasar activity across cosmic time (figure from Pizzati et al. 2024b)

Cosmological hydrodynamical simulations follow the non-linear evolution of structures in the Universe by modeling a large variety of processes that are important to the physics of galaxies, stars, and black holes. In the last decade, these simulations have become capable of reproducing a large number of observed galaxy properties. As a consequence, they have started to play a key role in shaping our understanding of the formation and evolution of galaxies in the universe, as well as the relation between galaxies and the supermassive black holes (SMBHs) that are harbored at their center. While much effort has been devoted to the study of the connection between SMBHs and their hosting galaxies in terms of gas fueling and AGN feedback, only few studies have focused on the global properties of SMBHs and their evolution over cosmic time. Specifically, SMBHs are visible at cosmological distances when they turn into bright quasars. Quasars are, however, extremely rare objects, and hence their statistical properties (such as their luminosity and spatial distributions) can only be studied theoretically when large simulated volumes are available. In a project that I am co-supervising, we use the cosmological simulations from the FLAMINGO suite (which are the largest ever run) to reproduce two basic observational probes of quasar activity: the quasar luminosity function and the quasar autocorrelation function. We study how well FLAMINGO can match observational constraints on these quantities at different redshifts. We also investigate which black holes contribute to the bright quasar population, and split their relative contribution in terms of black hole mass, host halo mass, and Eddington ratio. We are also interested in the relation between massive black holes and galaxies in FLAMINGO across cosmic time. Thanks to the state-of-the-art capabilities of FLAMINGO, for the first time we can probe the ability of large-volume hydrodynamical simulations to capture the evolution of massive black holes and quasar activity across cosmic time (figure from Schaye et al. 2023).

Investigating the complex environments of galaxies during the Epoch of Reionization is one of the most pressing research goals of modern astrophysics. As shown by cosmological simulations, galaxies are already at these epochs, and they present different properties with respect to the ones in the local Universe. Unraveling how galaxies formed and evolved during these remote epochs (i.e., a few hundred million years after the Big Bang) is at the heart of our understanding of cosmic evolution. In this context, the advent of telescopes such as ALMA and NOEMA have opened a new window on the primordial Universe, allowing us to shed light on the obscured star formation and ISM line emission at rest-frame FIR wavelengths up to z~7. One of the most compelling findings made by ALMA is that a significant fraction of z>4 galaxies is surrounded by extended (10-15 kpc) [C II]-emitting haloes that are not predicted by even the most advanced zoom-in simulations. This discovery poses a series of thorny theoretical questions, involving their formation mechanisms, evolution, and impact on the enshrouded galaxies as well as on the external regions where the intergalactic medium (IGM) resides. As part of my Master Thesis, I worked with Andrea Ferrara, Andrea Pallottini, and the cosmology group at SNS to take on the problem of a finding plausible mechanism to explain the formation of these halos. In particular, we focused on the hypothesis that these halos result from the remnants of past – or ongoing – outflow activity. We explored this idea by using a semi-analytical model for an outflow that undergoes catastrophic cooling in the inner region of the halo. Computing the abundance of singly ionized carbon and simulating the resulting [C II] emission, we compared it directly with observational evidence coming from recent ALMA data, and we conclude that outflows represent a promising answer to explain the origin of the observed [C II] halos. In a series of two papers, we analyzed the properties of [CII] halos in stacked and individual observations (from the ALMA ALPINE survey) and showed that the outflows that are needed to generate such halos have mass loading factors in the range ~4-7, and scale with stellar mass in a way that is consistent with the momentum-driven hypothesis. Our model points to the presence of extended [CII] emission at high redshifts as a tangible sign of star-formation-driven feedback mechanisms being already in place well into the Epoch of Reionization.

Constraining the strength of gas turbulence in protoplanetary discs is an open problem that has very relevant implications for the physics of gas accretion and planet formation. A promising method to gauge the level of gas turbulence in discs is to measure the vertical scale height of the dust component - which is expected to be coupled to the gas component through the gas-dust coupling. This has become possible in the last few years thanks to the very high-resolution observations provided by the ALMA telescope. These observations uncovered a large amount of features in the 2d images of protoplanetary discs, including dark gaps and emission rings. As shown by Pinte et al. 2016, it is possible to exploit these features to uncover the 3-d morphology of discs. The idea is very simple: due to projection effects, a gap in a disc emission profiles will be partly filled by the emission coming from the neighbouring regions. This effect is stronger along the minor axis of the disc, whereas the major axis is only marginally affected. For this reason, one can compare the gap contrast along the major and minor axes to infer the amount of "filling" that has taken place. The extent of this gap filling is directly dependent on the vertical structure of the disc. Hence, a measurement of the gap contrast along the major and minor axes can be used to infer the vertical scale height of the disc, and to indirectly gauge the amount of gas turbulence in discs. In a work done with Giovanni Rosotti and Benoit Tabone, we applied this method to the sample of discs observed by the DSHARP collaboration, for which high resolution images are available. We built a radiative transfer model to reproduce the observed gap constrast for different values of the dust scale heights. and found that the scale heights that yield a better agreement with data are generally low, implying low levels of gas turbulence in discs. This represents an important step towards understanding the physical processes that govern the formation of planets in discs. We also investigated the properties of the sources for which our method yields no significant constraints on the disc vertical structure, and concluded that this is because these discs have either a low inclination or gaps that are not deep enough. Based on our analysis, we provided an empirical criterion to assess whether a given disc is suitable to measure the vertical scale height (figure from Pizzati et al. 2023).

With the first direct detection of gravitational waves in 2015, the era of gravitational-wave astronomy has begun. This first decade of observations has already provided a wealth of information about compact binary mergers, neutron stars, and black holes. The future of this research field looks even brighter, with the next generation of detectors like the Cosmic Explorer and the Einstein Telescope expected to observe hundreds of thousands of binary coalescence events each year. With huge leaps in sensitivity, however, new challenges arise. One of the most pressing issues for the next generation of detectors is that signals from coalescing binaries will be so frequent that they will start to overlap with each other. When two signals overlap, the standard data-analysis pipelines used to detect and infer the properties of these signals are not guaranteed to work. While the detection pipelines have been shown to work well even in the presence of overlapping signals, the parameter inference algorithms used to estimate the properties of these signals represent a more compelling problem, as overlap of multiple signals in the time/frequency domains may result in biases in the final parameter estimates. In a work done with Bangalore Sathyaprakash, Surabhi Sachdev, Anuradha Gupta, and the LIGO group at Penn State, we were among the first to quantify the biases arising by using the current parameter inference pipelines to constrain the parameters of a compact binary coalescence (CBC) signal in the presence of multiple overlapping ones. We showed that, since the current detection pipelines provide the coalescence time of each signal with an accuracy of ~10 ms, one can set a prior on the coalescence time exploiting this information. In this way, it is possible to correctly infer the properties of multiple overlapping signals even with the current data-analysis infrastructure. By studying different configurations of overlapping signals, we find that the parameter inference is robust provided that the coalescence times of the signals in the detector frame are more than ~1-2 seconds apart. This implies that for the vast majority of the signals that will be detected by future detectors, our current framework for parameter inference will work just fine. Signals whose coalescence epochs lie within ~0.5 seconds of each other, however, suffer from significant biases in parameter inference, and new strategies and algorithms will be required in the future to overcome such biases (figure from Pizzati et al. 2022).