Jets and accretion in heavy black holes across cosmic time.

Blazars are Active Galactic Nuclei (AGN) characterized by relativistic jets launched in the vicinity of the central engine (i.e. a supermassive black hole; SMBH), that are oriented close to our line of sight. How jets in AGN form, collimate and accelerate is still an open issue, but a connection with the accretion process is sometimes suggested. We first investigated this issue, finding that in jetted AGN the SMBH can accrete both through a radiatively efficient, optically thick, geometrically thin accretion disc, and through a radiatively inefficient, geometrically thick hot accretion flow. The occurrence of these two accretion regimes depends on the accretion rate: if it is larger than ˙M ~ 0.1˙MEdd, the accretion is radiatively efficient, while if it is less than ~ 0.1˙MEdd the accretion is inefficient. After this first insight on accretion and jets, we used these components as tools to study the extremely massive black hole population at high redshift (i.e. MBH > 109M, z > 4). A deep knowledge of these objects can provide fundamental clues to the models of formation and growth of the first supermassive black holes (106M_ < MBH < 109M). The peculiar orientation of blazars makes them the most effective tracers of their parent population, namely all the jetted AGN with similar intrinsic properties, but oriented in random directions. For this reason, we set up a systematic search of blazar candidates from a large quasar sample, in order to collect a complete sample of high–redshift blazars. We selected a sample of 19 extremely radio–loud, high–redshift quasars. We characterized their nuclear features (i.e. SMBH mass and accretion rate) by fitting their accretion disc spectra: we find that our criteria are efficient in selecting very massive and fast accreting black holes. We started our classification campaign, through X–ray observations. We successfully classified three quasars from our sample as blazars, along with a serendipitously selected (but analogous) candidate from the same area of sky. This means that our criteria are efficient in selecting good blazar candidates. Comparing our findings with the known distribution of non–jetted AGN, we find that there are different formation epochs for extremely massive black holes hosted in jetted (z ~ 4) and non–jetted systems (z ~ 2.5). This is not easy to explain, according to the current black hole formation models in the early Universe. The search of very high–redshift jetted sources must be pursued, to learn more about the early stages of heavy black hole formation.