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

TL;DR

This study uses JWST/NIRSpec spectroscopy from CEERS, JADES, RUBIES, and GLASS to perform a 16-bin stacking analysis of ~125 galaxies per bin, probing broad H$\alpha$ emission as a tracer of BH activity down to $M_{BH} \sim 10^{5}$–$10^{6}\,M_\odot$ at $z\sim2$–6. The authors fit dual-component H$\alpha$ lines and find significant broad components in 5/16 stacks, with no corresponding broad [O III], indicating AGN-driven broad lines rather than outflows. BH masses inferred from these stacks ($\log(M_{BH}) \approx 5.2$–$6.1$) and host stellar masses ($\log(M_*) \approx 7.8$–$8.6$) imply that typical high-$z$ galaxies host BHs only modestly over-massive relative to the local $M_{BH}-M_*$ relation, consistent with light stellar-remnant seeds growing at moderate $f_{Edd}$; luminous individual JWST AGN likely sample the tail of the distribution. The results highlight the importance of stacking to overcome selection biases and to map the broader AGN population in the early universe, with implications for BH seeding and co-evolution scenarios. Overall, the work suggests that the median high-$z$ galaxy does not require exotic seeding to explain its BH mass, though a broader scatter in the relation likely persists.

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$α$ 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.

Figures (8)

• Figure 1: Continuum luminosity ($\nu L_{\nu}$) as a function of redshift for our complete galaxy sample ($\sim 2000$ galaxies). Continuum luminosities are measured from the reddest emission-line free NIRCam filter available for each source. CEERS sources are shown in red, JADES sources are shown in purple, GLASS sources are shown in pink, and RUBIES sources are shown in light blue. BL AGN identified in our sample are shown as stars and are removed from our stacking analysis.
• Figure 2: Median H$\alpha$ emission lines for our 16 stacks. Stacks increase in continuum luminosity from left to right, and increase in redshift from top to bottom. The total best-fit model is shown in purple for each stack, and a well-detected ($>3\sigma$) BL fit component is shown by the dashed red line. The $\rm{\Delta BIC}$, FWHM, and $\rm{L_{H\alpha, broad}}$ is given for each stack. Stacks that pass our three criteria for a detected BL, described in § \ref{['sec: Emission Line Fitting']}, have their $\rm{\Delta BIC}$, FWHM, and $\rm{L_{H\alpha, broad}}$ shown in red and have a white background. Stacks that do not pass the BL identification criteria have a light gray background.
• Figure 3: Median [O iii ] emission lines for our 16 stacks. Stacks increase in continuum luminosity from left to right, and increase in redshift from top to bottom. The total best-fit model is shown in purple for each stack, and a well-detected ($>3\sigma$) BL fit component is shown by the dashed red line. The $\rm{\Delta BIC}$, FWHM, and $\rm{L_{H\alpha, broad}}$ is given for each stack. Stacks that pass our three criteria for a detected BL, described in § \ref{['sec: Emission Line Fitting']}, have their $\rm{\Delta BIC}$, FWHM, and $\rm{L_{H\alpha, broad}}$ shown in red. The lack of BL H$\alpha$ detections indicates evidence for no outflows in these stacks. Stacks that do not pass the BL identification criteria have a light gray background.
• Figure 4: Flux amplitude and FWHM posteriors from our stacked H$\alpha$ fits. Stacks with detected broad H$\alpha$ emission have their best fit flux and FWHM measurements shown with the pink star. Full BH mass posteriors are obtained by computing masses with the full flux and FWHM posteriors. BH mass contours of $10^5~\rm{M_{\odot}}$ and $10^6~\rm{M_{\odot}}$ are shown with the dashed black lines. The BH mass prescription we use is discussed in § \ref{['sec: bh masses']}.
• Figure 5: BH mass as a function of redshift. The broad H$\alpha$ line detections from our stacks are shown with the red stars, and $3\sigma$ upper-limits for the non-detections are shown by the open stars. Two large JWST BL AGN surveys are shown with the light blue circles Taylor2024 and the purple pentagons Juodzbalis2025. The highest and second-highest [O iii ] luminosity stacks from Geris2025 are shown with the cyan squares. We show the growth of a heavy $10^{4-5}M_{\odot}$ BH seed with $f_{Edd} = 0.1$ (gray shading), which is the typical Eddington ratio at lower redshifts Ananna2022. We show three growth paths of a Population III stellar remnant ($10^2 M_{\odot}$) with $f_{Edd} = 1$ (solid black line), $f_{Edd} = 0.5$ (dashed black line), and $f_{Edd} = 0.1$ (dotted black line). In our models, the stellar remnant formed at $z = 30$, with accretion beginning after a 100 Myr delay.