Understanding the formation of the first black holes: deep observations of the most distant quasars

The existence of supermassive black holes powering quasars up to z ∼ 7.5 is a challenge for modern astrophysics. Recent simulations suggest that the most massive SMBHs at z ∼ 6 can only form within the densest regions of the Universe that would possibly later evolve into local massive galaxy clusters, but evidences of high-z overdensities are still unclear. Assessing the cosmic evolution and environment of the AGN population up to very high redshifts is a key factor to understand and constrain the early growth of SMBHs, but it requires the collection of large samples of AGN. In this thesis work we provide a systematic analysis and study of all the X-ray data available for z ∼ 6 QSOs in order to derive the general properties of accretion of SMBHs in the early Universe. Comparing the results obtained from this X-ray analysis with those derived from past X-ray observations, we discovered the first example of an X-ray variable QSO at z > 6. Deep and wide imaging observations of a region around this object showed that this field features the best evidence to date for an overdense region around a z ∼ 6 QSO. To confirm that, we obtained deep Chandra observations to build a catalog of X-ray detected sources with multi-wavelength information that will be used in future works to search and find possible obscured/faint AGN at the QSO redshift. This quest for overdense regions around early SMBHs was also complemented by an optical/NIR search for large-scale QSO companions around 75 known z ∼ 6 QSOs. From this study we found 25 good QSO companion candidates that will be followed-up with future spectroscopic observations.