The Search for the Farthest Quasar: Consequences for Black Hole Growth and Seed Models

Abstract

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 $f_{\mathrm{Edd}}$, duty cycle $\mathcal{D}$, seed mass $M_{\bullet ,\mathrm{seed}}$ and radiative efficiency $\epsilon$), we show that the large uncertainties in their determination decrease by a factor $\sim 5$ when a quasar's detection redshift goes from $z=9$ to $z=12$. In this high-redshift regime, $\epsilon$ tends to the lowest value allowed, and the distribution for $M_{\bullet ,\mathrm{seed}}$ 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 $M_{\bullet ,\mathrm{seed}}>10^{3.5}{\mathrm{M}}_{\odot }$ and $\epsilon <0.1$. The radiative efficiency is a crucial unknown, with factor $\sim 2$ changes able to modify the predicted mass by $\sim 3$ orders of magnitude already at $z\sim 9$. The competing roles of inefficient accretion (decreasing $\epsilon$) and black hole spin-up (increasing $\epsilon$) 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-disk accretion, a detection of a quasar with $M_{\bullet }\sim 10^{10}{\mathrm{M}}_{\odot }$ by $z\sim 9-10$ would exclude the entire parameter space available for light seeds and dramatically reduce the one for heavy seeds.

Accepted for publication in MNRAS. This is the final version of the manuscript. 8 pages, 5 figures

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