Redshift distribution of the GDR. The symbols and colors for the WISSH and our sample are the same as in Fig. 12. We compare our results with the WISSH sample (Bischetti et al. 2021), a sample of 2 < z < 5 star-forming galaxies hosting a heavily obscured AGN in the Chandra Deep Field-South (magenta dots, D’Amato et al. 2020), and a sample of z > 5.5 QSOs (blue squares Calura et al. 2014). The two vertical lines at the bottom right side are the systematic uncertainties induced by the choice of α_CO and dust mass estimation (black line, ∼0.3 dex) and r₆₅ or r₇₆ in computing the gas mass (brown line, ∼0.1 dex).