Beyond the Monsters: A More Complete Census of Black Hole Activity at Cosmic Dawn

TL;DR

This study uses JWST spectroscopy to perform a comprehensive stacking analysis of high-redshift galaxies, revealing that most black holes in early galaxies are only modestly over-massive and follow local universe relations, challenging previous claims of overly massive black holes.

Contribution

It provides the first detailed census of black hole populations at cosmic dawn using stacking techniques, showing they align closer to local relations than previously thought.

Findings

Median black hole masses are between 10^5.21 and 10^6.13 solar masses.

Most galaxies host black holes within a factor of 10 of local mass relations.

Black hole seeding can be explained by light stellar remnants with moderate accretion.

Abstract

JWST has revealed an abundance of low-luminosity active galactic nuclei (AGN) at high redshifts ($z > 3$), pushing the limits of black hole (BH) science in the early Universe. Results have claimed that these BHs are significantly more massive than expected from the BH mass-host galaxy stellar mass relation derived from the local Universe. We present a comprehensive census of the BH populations in the early Universe through a detailed stacking analysis of galaxy populations, binned by luminosity and redshift, using JWST spectroscopy from the CEERS, JADES, RUBIES, and GLASS extragalactic deep field surveys. Broad H$\alpha$ detections in $31\%$ of the stacked spectra (5/16 bins) imply median BH masses of $10^{5.21} - 10^{6.13}~ \rm{M_{\odot}}$ and the stacked SEDs of these bins indicate median stellar masses of $10^{7.84} - 10^{8.56} ~\rm{M_{\odot}}$. This suggests that the median galaxy hosts a BH that is at most a factor of 10 times over-massive compared to its host galaxy and lies closer to the locally derived $M_{BH}-M_*$ relation. We investigate the seeding properties of the inferred BHs and find that they can be well-explained by a light stellar remnant seed undergoing moderate Eddington accretion. Our results indicate that individual detections of AGN are more likely to sample the upper envelope of the $M_{BH}-M_*$ distribution, while stacking on ``normal" galaxies and searching for AGN signatures can overcome the selection bias of individual detections.