2020MNRAS.496..888N

Supermassive black holes (SMBHs) play a crucial role in the evolution of galaxies and are currently detected up to z ∼ 7.5. Theories describing black hole (BH) growth are challenged by how rapidly seeds with initial mass M_•≲10⁵ M_☉, formed at z ∼ 20-30, grew to M_•∼10⁹ M_☉ by z ∼ 7. Here we study the effects of the value of the Hubble parameter, H₀, on models describing the early growth of BHs. First, we note that the predicted mass of a quasar at z = 6 changes by > 300 per cent if the underlying Hubble parameter used in the model varies from H₀ = 65 to H₀ = 74 km s^–1Mpc^–1, a range encompassing current estimates. Employing an MCMC approach based on priors from z >= 6.5 quasars and on H₀, we study the interconnection between H₀ and the parameters describing BH growth: seed mass M_i and Eddington ratio f_Edd. Assuming an Eddington ratio of f_Edd = 0.7, in agreement with previous estimates, we find H₀=73.6^+1.2_–3.3 km s^–1Mpc^–1. In a second analysis, allowing all the parameters to vary freely, we find log (M_i/M_☉) > 4.5 (at 95 per cent CL), H₀= 74^+1.5_–1.4 km s^–1Mpc^–1 and f_ Edd_=0.77+0.035^_–0.026 at 68 per cent CL. Our results on the typical Eddington ratio are in agreement with previous estimates. Current values of the Hubble parameter strongly favour heavy seed formation scenarios, with M_i≳10⁴ M_☉. In our model, with the priors on BH masses of quasars used, light seed formation scenarios are rejected at ∼3σ.