Blazars in the re-ionization epoch

Search and Study of Blazars in the early Universe PhD thesis by Silvia Belladitta , Matr. 733846 The aim of this thesis is to identify and characterize blazars at z>4, to provide more constraints on the properties of high-z radio-loud (RL) Active Galactic Nuclei (AGNs) population. Blazars are RL AGNs with relativistic jets pointing towards the Earth. The discovery of high-z blazars ensures the census, free from obscuration effects, of supermassive black holes (SMBHs) hosted by RL AGNs in the early Universe. Indeed for each observed blazar we expect to find ~2Γ2 (where Γ is the bulk Lorentz factor of the jet) RL AGNs, whose jets point in other directions, with the same properties (e.g. MBH, Lradio, etc.). By combining different radio/optical/infrared surveys available to date and by using the dropout method I built up a statistically complete sample (21 objects) of blazars at 4<5.5, and I discoverer new (3) blazars at z>5. One of these, at z=6.1, is also the most distant blazar ever discovered. With these sources I increased by 50% the number of blazars known to date at z>4. I performed a multi wavelength analysis of the properties of all these sources (from radio to X-ray bands) and I compared them to both RL and radio-quiet (RQ, AGNs without relativistic jets) AGNs at similar redshift. The blazars reported in this thesis are all powerful X-ray sources, have a similar radio spectrum (flat or peaked) and host SMBHs with MBH>108M⊙. The presence of such massive objects at high-z requires possible super-eddington accretion episodes for the seed black holes growth.