2022MNRAS.509.1885P

The quest for high-redshift quasars has led to a series of record-breaking sources, with the current record holder at z = 7.642. Here, we show how future detections of z > 8 quasars impact the constraints on the parameters for black hole growth and seed models. Using broad flat priors on the growth parameters (Eddington ratio f_Edd, duty cycle D, seed mass M_*,seed, and radiative efficiency ε), we show that the large uncertainties in their determination decrease by a factor ∼5 when a quasar's detection redshift goes from z = 9 to z = 12. In this high-redshift regime, ε tends to the lowest value allowed, and the distribution for M_*,seed peaks well inside the heavy seed domain. Remarkably, two quasars detected at z > 7 with low accretion rates (J1243+0100 and J0313-1806) already tighten the available parameter space, requiring M_•, seed_ > 10^3.5 M_☉ and ε < 0.1. The radiative efficiency is a crucial unknown, with factor ∼2 changes able to modify the predicted mass by ∼3 orders of magnitude already at z ∼ 9. The competing roles of inefficient accretion (decreasing ε) and black hole spin-up (increasing ε) significantly impact growth models. Finally, we suggest that yields currently predicted by upcoming quasar surveys (e.g. Euclid) will be instrumental for determining the most-likely seed mass regime. For example, assuming thin-disc accretion, a detection of a quasar with M_•∼_10¹⁰ M_☉ by z ∼ 9-10 would exclude the entire parameter space available for light seeds and dramatically reduce the one for heavy seeds.