Black hole mass evolution as a function of time (or redshift, upper axis), for the results obtained with the SLIMBH-based approach (consistent with KERRBB-based results). The data point shows 1508+5714 virial mass and the relative mass range obtained, as seen in Sect. 3.3. The solid blue line and light blue shaded area show how the mass would evolve if it followed the same accretion regime as derived with SLIMBH, starting from z = 20. The derived seed black hole masses, M_s, are labeled. The green shaded area, instead, follows an Eddington-limited evolution in the case of a radiative efficiency ranging between 5 and 10% (dotted and dashed lines, respectively). Following this accretion regime, seed black holes in the range 50−200 M_⊙ are inferred between z = 5.5 and z = 8, respectively, as labeled.