The Lumina Project: The Demographics of Active Galactic Nuclei from Quasars to Little Red Dots at z ≥ 3

Abstract High-redshift active galactic nuclei (AGN) are powerful probes of early black-hole growth, galaxy formation, and the evolving intergalactic medium (IGM). We use Lumina, a cosmological radiation-hydrodynamic simulation spanning the epochs of hydrogen and helium reionization, which combines a large (500 cMpc)3 volume with 2 × 60003 resolution elements, to explore high-redshift AGN. The simulation self-consistently follows hundreds of millions of galaxies and supermassive black holes (SMBHs), together with their impact on the IGM. We exploit this uniquely large dynamic range to predict multi-band AGN luminosity functions (LFs) at z ≥ 3, from hard X-rays to the mid-infrared. These predictions encompass both moderately luminous quasars and the faint “Little Red Dots” (LRDs) uncovered by JWST. We develop an empirical model that maps simulated SMBHs onto observed AGN using bolometric and extinction/absorption corrections for canonical AGN and LRDs, and in which SMBHs with $M_{\rm BH}\le 10\, M_{\rm seed} \sim 10^{7}{\, \rm M_\odot }$ stay in the LRD phase with a duty cycle of 30%. This simple framework reproduces the observed LFs and clustering of LRDs. Meanwhile, the pre-JWST quasar LF constraints are recovered, although we find that a ∼0.3 dex log-normal scatter in bolometric luminosity is required to reproduce the bright end. We quantify the redshift evolution of AGN and LRD number densities, and their contributions to the integrated BH mass densities. The same AGN population is the dominant driver for the He ii reionization modelled self-consistently in Lumina. This paves the way for general population-synthesis models of high-redshift AGN, including LRDs, in a unified cosmological framework.