Gravitational waves throughout galaxy evolution: stellar BH mergers and heavy SMBH seeds.

The main goal of my thesis is to carefully characterize different astrophysical processes leading to gravitational wave (GW) emission, strongly relying on theoretical and observational astrophysical basis. From an observational point of view, current interferometers (Advanced Laser Interferometer Gravitational wave Observatory/Virgo (AdvLIGO/Virgo)) and future detectors (Einstein Telescope (ET), Cosmic Explorer (CE), Deci-hertz Interferometer Gravitational wave Observatory (DECIGO), Laser Interferometer Space Antenna (LISA)) will greatly enlarge the number of detected GW events. However, in order to extract meaningful information about various astrophysical phenomena and improve our knowledge on cosmology and fundamental physics from this large sample of observational data, a correct modelization of the impact of different astrophysical processes on GWs rates is necessary. The marking feature of all the work is an accurate and deep study of the galactic environment, making use of classic theoretical arguments and recent observational results in the galaxy formation and evolution field. Galactic properties, such as star formation rate, gas and stellar density, metallicity, can have a profound impact on stellar and compact object evolution and on the ensuing GW emissions. In particular, throughout the thesis I focused on the study of 2 different channels of GW production: merging of isolated double compact object binaries of stellar origin (neutron stars and stellar black holes) and dynamical merging of stellar and, eventually, primordial black holes in the central regions of early-type galaxy progenitors. In the context of double compact object merging binaries, given the relevance of gas-phase metallicity for all the stellar and binary evolution processes, the main effort of my work is in the characterization of a metallicity dependent cosmic star formation rate density. I compute this term in various ways, highlighting the impact of different galactic prescriptions, such as galaxy statistics and metallicity scaling relations. In particular I focus on the gas-phase metallicity, showing that the two main empirical scaling relations present in literature, the Mass Metallicity Relation and the Fundamental Metallicity Relation, hold substantially different results at high redshift ( > 2), with the Fundamental Metallicity Relation featuring relatively high metallicitites ∼ 0.4 − 0.5 Z⊙ and the Mass Metallicity Relation predicting a significant metallicity drop below 0.1 Z⊙. I discuss the reasons and possible biases originating this discrepancy, arguing in favor of the Fundamental Metallicity Relation or of a slowly declining Mass Metallicity Relation. I also present a chemical evolution model to deal with metallicity from a theoretical point of view and I find a pleasant agreement between the model and the Fundamental Metallicity Relation. Finally, I show the impact of these different astrophysical prescriptions on the merging rates and on the properties of compact objects binaries, such as their chirp mass or time delay distribution. I complete the work forecasting the ensuing GW detection rates with present and future detectors, as well as the expected lensed event rates and the stochastic GW background. As for the dynamical merging channel, recent observations of the extremely star-forming and gas-dense environments in the central regions of early-type galaxy progenitors at z bigger than 1, inspired the idea for the proposal of a new mechanism for the growth of supermassive black hole seeds. This envisages the migration and merging of compact objects via gaseous dynamical friction toward the galactic center where a central black hole accumulates mass thanks to these continuous merging events. I show that, under reasonable assumptions, the process can build up central BH masses of order 10^4 − 10^5 M⊙ within some 10^7 yr, so effectively providing heavy seeds before standard (Eddington-like) disk accretion takes over to become the dominant process for further BH growth. Remarkably, such a mechanism may provide an explanation, alternative or complementary to other processes, for the buildup of billion solar masses black holes in quasar hosts at z bigger than 7, when the age of the Universe less than 0.8 Gyr constitutes a demanding constraint. This process naturally present a possibility to be tested via detections of the gravitational waves produced by mergers between the migrating compact objects and the growing central black hole. I also make predictions for the produced stochastic GW background which extends over a wide range of frequencies [10^(−6) Hz, 10 Hz], very different from the typical range originated by mergers of isolated binaries. I show that both the single events and the background could be revealed by future ground- and space-based interferometers as ET, DECIGO and LISA.