PREPRINT
392E20D8-A907-4BAC-BC08-951A18A6A8F6

# The need for obscured supermassive black hole growth to explain quasar proximity zones in the epoch of reionization

Sindhu Satyavolu, Girish Kulkarni, Laura C. Keating, Martin G. Haehnelt

Submitted on 16 September 2022

## Abstract

Proximity zones of quasars with redshifts $z\gtrsim 6$ are unique probes of the growth of supermassive black holes. But simultaneously explaining proximity zone sizes and black hole masses at this redshift has proved to be challenging because of the very short quasar lifetimes implied by the proximity zones. We study the robustness of some of the assumptions that are usually made to infer quasar lifetimes from proximity zone sizes. We show that thanks to the short equilibration time of gas inside the proximity zones, small proximity zones can be readily explained by quasars that vary in brightness with a short duty cycle of ${f}_{\mathrm{duty}}\sim 0.1$ and short bright periods of ${t}_{\mathrm{on}}\sim {10}^{4}$ yr, even for long lifetimes. We further show that reconciling this with black hole mass estimates requires the black hole to continue to grow and accrete during its obscured phase. The consequent obscured fractions of $\gtrsim$ 0.7 or higher are consistent with low-redshift measurements and models of black hole accretion. Further, the large dynamic range of our simulation, and its calibration to the Lyman-$\alpha$ forest, allows us to investigate the influence of the large-scale topology of reionization and the quasar's host halo mass on proximity zones. We find that incomplete reionization can impede the growth of proximity zones and make them smaller up to 30%, but the quasar host halo mass only affects proximity zones weakly and indirectly. Our work suggests that high-redshift proximity zones can be an effective tool to study quasar variability and black hole growth.

## Preprint

Comment: 19 pages, 17 figures, submitted to MNRAS, comments welcome

Subjects: Astrophysics - Astrophysics of Galaxies; Astrophysics - Cosmology and Nongalactic Astrophysics