Black Hole Accretion in Primordial Galaxies

Supermassive black holes (SMBHs) with masses up to 1010M⊙ are thought to power the emission from quasars and Active Galactic Nuclei. Surprisingly, these extremely massive, compact objects are already in place within the first billion years from the Big Bang, or redshift z ≥ 6. In addition, a tight relation between the stellar and central black hole mass in galaxies has been established locally. The SMBH origin, evolution and relation to the host galaxy are key open problems in modern cosmology and astrophysics. They are the main subject of this Thesis. As far as the origin is concerned, the most natural idea considers SMBHs as the end-product of accretion and merging on black hole remnants of massive stars. We have examined this possibility showing that this hypothesis entails several theoretical difficulties. To this aim, we have followed the accretion history of a 100M⊙ stellar BH hosted by a typical z = 10 galaxy down to z = 6. We analysed the growth under different conditions linked to the galaxy geometry and BH orbital parameters. We conclude that in all cases, the BH mass can increase at most by 30%, thus making stellar seeds unsuitable to explain the observed masses of SMBH. As a by-product of the study, we have estimated the cumulative X- ray emission from an early BH population and the total energy released in the intergalactic medium. Given the above low accretion rates, we conclude that the X-ray emission from accreting BHs is negligible with respect to that provided by X-ray binaries in the same galaxy. Although sub-dominant, the X-ray preheating of the intergalactic medium by early BHs might have left a specific signature, potentially detectable with SKA, on the HI 21cm line power spectrum. The above results forced us to look for alternative SMBH formation processes. Thus, we explored the scenario in which SMBHs grow by merging of more mas- sive seeds, directly formed through non-stellar channels. These theoretically pre- dicted, massive (104 − 106M⊙) “direct collapse” seeds provide a head-start of the SMBH build-up and overcome the inefficient stellar BH accretion. Lacking an observational confirmation of the existence of these putative SMBH progeni- tors, we first assessed whether these ancestors can be detected by the Chandra X-ray Observatory, and used the upper limits provided by current observations to constrain theoretical model parameters. For this purpose, we exploit a semi- analytical model to predict the number density of progenitors of a z = 6.4 SMBH, their accretion and the amount of obscuring material in their host galaxies. For each ancestor we computed its X-ray spectrum accounting for interstellar absorp- tion and compared it with current observations. Faint progenitors are found to be luminous enough to be detected in the X-ray band of current surveys. Even accounting for a maximum obscuration effect, the number of detectable BHs is reduced at most by a factor of 2. In our simulated sample, observations of faint quasars are mainly limited by their very low active fraction (about 1%), which is the result of short, super-critical growth episodes. We suggest that to detect high-z SMBHs progenitors, large area surveys with shallower sensitivities, such as COSMOS Legacy and XMM-LSS+XXL, are to be preferred with respect to deep surveys probing smaller fields, such as Chandra Deep Field South. The models discussed so far imply that massive black holes (≈ 108M⊙) must be present also in galaxies routinely observed in the Epoch of Reionization, such as the so-called Lyman Break Galaxies (LBG) at z > 6. This is an important point because the presence of a (faint) AGN in these systems might substantially alter their physical and observable properties. We addressed this question by combining our semi-analytical model with tight constraints from the 7 Ms Chan- dra survey, and the known high-z SMBH population. Depending on the fraction of early halos planted with a BH direct collapse seed, the model suggests two possible scenarios: (a) if a maximal seeding occurs, massive BH in LBGs mostly grow by merging and must accrete at a low Eddington ratio not to exceed the ex- perimental X-ray luminosity upper bound; (b) if the seeding is inefficient, direct accretion dominates and massive BH emission in LBGs must be heavily obscured. Scenario (a) poses extremely challenging, and possibly unphysical, requirements on seeds formation. Scenario (b) entails testable implications on the physical properties of LBGs involving far-infrared luminosity, emission lines, and presence of outflows that we discuss in detail.