High-redshift AGN population in radiation-hydrodynamics simulations

ABSTRACT High-redshift active galactic nuclei (AGNs) have long been recognized as key probes of early black hole growth and galaxy evolution. However, modelling this population remains difficult due to the wide range of luminosities and black hole masses involved, and the high computational costs of capturing the hydrodynamic response of gas and evolving radiation fields on the fly. In this study, we present a new suite of simulations based on the IllustrisTNG galaxy formation framework, enhanced with on-the-fly radiative transfer, to examine AGN at high redshift ($z \gtrsim 5$) in a protocluster environment extracted from the MillenniumTNG simulation. We focus on the coevolution of black holes and their host galaxies, as well as the radiative impact on surrounding intergalactic gas. The model predicts that black holes form in overdense regions and lie below the local black hole–stellar mass relation, with stellar mass assembly preceding significant black hole accretion. Ionizing photons are primarily produced by stars, which shape the morphology of ionized regions and drive reionization. Given the restrictive black hole growth in the original IllustrisTNG model, we reduce the radiative efficiency from 0.2 to 0.1, resulting in higher accretion rates for massive black holes, more bursty growth, and earlier AGN-driven quenching. However, the resulting AGNs remain significantly fainter than observed high-redshift quasars. As such, to incorporate this missing population, we introduce a quasar boosted model, in which we artificially boost the AGN luminosity. This results in strong effects on the surrounding gas, most notably a proximity effect, and large contributions to He ionization.