The Formation of Supermassive Black Holes From Population III.1 Seeds – Implications for Clustering, Binarity and Gravitational Waves

The origin of supermassive black holes (SMBHs) remains an open question in astrophysics. The presence of black holes more massive than a billion solar masses at high redshifts (z>6) challenges many formation mechanisms. One mechanism that can explain these SMBHs invokes the collapse of Pop III.1 stars as seeds of SMBHs. These are the Population III stars forming in dark matter mini halos in the early universe, but with the additional condition of isolation from other stellar or SMBH feedback sources (McKee & Tan 2008). This isolation condition, parameterized by an isolation distance, d_iso, leads to limited fragmentation and thus co-location of the star with the dark matter cusp. If the dark matter particles in the halo are assumed to be Weakly Interacting Massive Particles (WIMPs), then their self-annihilation inside the protostar would release energy which can be captured by the star, allowing it to maintain a relatively cool photosphere (Spolyar et al. 2008; Freese et al. 2010; Rindler-Daller et al. 2015). This results in low levels of ionizing feedback, thus more efficient accretion of almost the entire baryonic content of its natal minihalo, i.e. ~10^5 M_sun. In Banik, Tan and Monaco (2019), this seeding mechanism was applied on a cosmological box of (40 Mpc/h)^3 using the PINOCCHIO code (Monaco et al. 2002; Munari et al. 2017) and the evolution of the seeded halos was followed down to z=10. The number density of SMBHs at that redshift matches the estimated co-moving number density at z=0 for values of d_iso~100 kpc (proper distance). In this thesis, we investigate the Pop III.1 seeding mechanism in PINOCCHIO simulations of (40 Mpc/h)^3 volumes and follow the evolution to the local universe at z = 0. We present the methods and tools developed to identify and track the evolution of seeded halos. With these tools, we compute the evolution of the number density of seeded halos and the occupation fraction of halos. We also compute the correlation functions of the seeded halos, while also accounting for the finite size of the simulation box. The clustering signal shows good agreement with the observations of luminous galaxies in the local universe, and we predict a distinct drop in the clustering at high redshifts at small scales corresponding to the size of the isolation sphere during the formation epoch of the seeds. Then we present a method to distinguish among different seeding mechanisms by creating synthetic Ultra Deep Fields of the same size as the Hubble Ultra Deep Field. In the next part of the thesis, we compute the binary statistics of SMBHs within dark matter halos as a function of redshift and for different values of isolation distance. We also investigate the effect on these statistics of different prescriptions for galaxy and SMBH merger timescales. We then compare our results to the latest observations of AGN multiplicity such as the Subaru Strategic Program (Aihara et al. 2018, 2019; Silverman et al., 2020). In the final part of the thesis, we compute the gravitational wave background (GWB) emanating from SMBH binaries produced via the Pop III.1 seeding mechanism. Since PINOCCHIO outputs the merger history of the halos, we extract the mergers of seeded halos and assume a time delay between the merger of the halos and the SMBHs occupying them. Then, to calculate the merger rate of the black holes as a function of redshift and chirp mass, we compute the average rate of seeded halo mergers from 10 PINOCCHIO boxes. To calculate the mass of the SMBHs inhibiting the seeded halos, we use halo mass – black hole mass scaling relations. Then we finally compute the value of the GWB and explore the impact of different assumptions of time delay and the mass scaling relations. Finally, we do comparisons with the latest results from Pulsar Timing Array experiments, particularly the NANOGrav collaboration (Agazie et al. 2023).