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

TL;DR

This study uses high-redshift galaxy simulations and accretion models to explain the abundance of observed faint active galactic nuclei, suggesting they are super Eddington accreting black holes in low-mass galaxies.

Contribution

It demonstrates that current models can account for the observed high-redshift AGN population and proposes a scenario involving super Eddington accretion in low-mass galaxies.

Findings

Models predict sufficient AGN abundance to match observations.

Overprediction of low-luminosity AGN suggests super Eddington accretion in small galaxies.

Mock observations replicate key properties of observed faint AGN.

Abstract

The James Webb Space Telescope has unveiled an abundant population of potential active galactic nuclei (AGN) at high redshift ($z\gtrsim4$) 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 free-fall accretion model to derive black hole accretion rates and construct predicted AGN bolometric luminosity functions for $z=5-7$. The GTDA model and the free-fall model with black holes accreting $\lesssim 1$ percent of their central gas supply ($<100 \rm \ pc$) per free-fall 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 and which also provides a good match to the host-galaxy UV luminosity distribution suggests that LRDs are super Eddington-accreting, Eddington luminosity-limited, $M_{\rm BH}\gtrsim 2\times10^5 \ \rm M_\odot$ black holes residing in $M_\star\gtrsim 2\times10^7 \ \rm M_\odot$ galaxies. We show that, under simple assumptions, mock observations of such sources can reproduce key observed LRD characteristics.