Chasing the observational signatures of seed black holes at z > 7: Candidate statistics

Supermassive black holes (SMBHs) of 109–1010M were already in place ∼13 Gyr ago, at z > 6. Super-Eddington growth of low-mass BH seeds (∼100M) or less extreme accretion on to∼105M seeds have been recently considered as the main viable routes to these SMBHs. Here, we study the statistics of these SMBH progenitors at z ∼ 6. The growth of low- and high-mass seeds and their host galaxies are consistently followed using the cosmological data constrained model GAMETE/QSODUST, which reproduces the observed properties of high-z quasars, like SDSS J1148+5251.We show that both seed formation channels can be in action over a similar redshift range 15 < z < 18 and are found in dark matter haloes with comparable mass, ∼5 × 107M. However, as long as the systems evolve in isolation (i.e. no mergers occur), noticeable differences in their properties emerge: At z ≥ 10 galaxies hosting high-mass seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star formation proceeds less efficiently than in low-mass seeds hosts. At z < 10 these differences are progressively erased, as the systems experience minor or major mergers and every trace of the BH origin gets lost.