galaxy requires the presence of a large amount of gas. In fact, infrared sky surveys have discovered huge amounts of molecular gas (CO) at intermediate and high-z QSOs (Downes et al 1999; Bertoldi et al 2003; Weiss et al 2007). In particular, the detection of CO emission in the quasar J1148+5251 at $z = 6.42$ permitted an estimation of the molecular hydrogen mass present in the central region of the host galaxy that amounts to $M(H_2) \sim 10^{10}~M_\odot$ (Walter et al. 2009). Moreover, at least in the case of the quasar J1319+0950 ($z = 6.13$) there is a robust evidence that the gas is rotating (Shao et al. 2017), suggesting the presence of a gaseous disk. More recently, observations of J1342+0928 ($z = 7.54$) indicate important amounts of gas and dust revealed by the infrared continuum and by the \[CII\] line emission (Venemans et al. 2017). All these observations support the idea that some massive galaxies in their early evolutionary phases had large amounts of gas in their central regions, which could have formed the massive accretion disks required by our model.

If observations seem to support the scenario in which massive disks fed the seeds of SMBHs, the other 