universe. Their luminosities are typically around $10^{46} erg.s^{-1}$ but the emission of some QSO’s may exceed by one or two orders of magnitude that value. Edwin Salpeter was one of the first to propose that a supermassive black hole (SMBH) in a state of accretion could provide the necessary energy to explain the luminosities of QSOs (Salpeter 1964). If presently a large majority of the scientific community accepts that accreting SMBHs are the engines powering QSOs, a series of questions still remain to be answered. For instance, how these SMBHs are formed? If they grow by accretion what are the seeds and from where they come from? What is the gas accretion geometry: spherically or that of an inspiraling disk? In last case, what are the viscous mechanisms responsible for the transfer of angular momentum?

In the local universe the presence of SMBHs in the center either of elliptical galaxies or bulges of spiral galaxies seems to be a well-established fact (Kormendy & Richstone 1995; Richstone et al. 1998; Kormendy & Gebhardt 2001). The black hole mass $M$ is well correlated either with the stellar mass or the luminosity of the host bulge (Kormendy & Richstone 1995; Magorrian et al. 