Little Red Dots on FIRE: The Ability of Bursty Galaxies to Host an Abundant Population of High-redshift AGN

Abstract The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift ( z ≳ 4) known as little red dots (LRDs), which are likely hosted in relatively low-mass galaxies. However, previous theoretical models have highlighted the difficulty in continuously feeding massive black holes in the central regions of bursty, high-redshift galaxies because of repeated gas evacuation by stellar feedback. We analyze galaxies in high-redshift FIRE-2 simulations to understand whether they are capable of hosting the observed abundant population of high-redshift AGN. We use a gravitational torque-driven accretion (GTDA) model and a simple freefall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for z = 5–7. The GTDA model and the freefall model with black holes accreting ≲1% of their central gas supply (<100 pc) per freefall time predict AGN abundances that are more than sufficient to explain the most recent LRD observations. The fiducial models, in fact, overpredict the number of low-luminosity AGN as compared with observations. We explore possible resolutions of this tension. A plausible, though likely not unique, scenario for alleviating the AGN overpredictions, which also provides a good match to the host-galaxy UV luminosity distribution, suggests that LRDs are super-Eddington-accreting, Eddington-luminosity-limited, M BH ≳ 2 × 10 5 M ⊙ black holes residing in M ⋆ ≳ 2 × 10 7 M ⊙ galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.