A prevalent population of normal-mass central black holes in high-redshift massive galaxies

Junyao Li, Yue Shen, Ming-Yang Zhuang

First listed 2025-02-07 | Last updated 2025-02-07

Abstract

Understanding the co-evolution between supermassive black holes (SMBHs) and their host galaxies provides crucial insights into SMBH formation and galaxy assembly in these cosmic ecosystems. However, measuring this co-evolution, as traced by the black hole mass - stellar mass relation towards the early Universe, often suffers from significant sample selection biases. Samples selected based on the luminosity of the SMBH would preferentially find overly massive black holes relative to their host stellar mass, missing the population of lower-mass SMBHs that are underrepresented. Here we report the discovery of 13 moderate-luminosity broad-line Active Galactic Nuclei from a galaxy-based selection of 52 massive galaxies at z~3-5. The derived SMBH masses for these AGNs yield a mean SMBH-to-stellar mass ratio of ~0.1%, consistent with the local value. There is limited evolution in this mean mass ratio traced back to z~6, indicating that a significant population of ''normal'' SMBHs already existed within the first billion years of the Universe. Combined with the previous sample of overmassive black holes, there must be diverse pathways for SMBH formation in high-redshift galaxies. Most of these galaxies are experiencing star formation quenching by the observed epoch, suggesting the formation of massive quiescent galaxies does not necessarily require an overly massive black hole, contrary to some theoretical predictions.

Short digest

From a galaxy-selected parent sample of 52 massive systems at z≈3–5, the authors use JWST/NIRSpec medium–high resolution spectra to search the Hα+[N II]+[S II] region for broad components (FWHM>1000 km/s), identifying 13 moderate-luminosity BLAGNs. Virial Hα masses and AGN luminosities place these sources on the local M_BH–M_* relation with an average M_BH/M_*≈0.1%, contrasting with previously JWST-selected overmassive LDs and quasars. Back-tracing indicates limited evolution in this mean ratio to z≈6, implying that a prevalent population of “normal” SMBHs was already in place within the first Gyr. Host SEDs show prominent Balmer/4000 Å breaks and red continua, suggesting many of these massive galaxies are already quenching, so assembling massive quiescents need not require overmassive black holes.

Key figures to inspect

• Figure 1: Inspect where the 13 BLAGNs land in the M_BH–M_* plane versus LDs and luminous quasars; the tight locus around the local relation (color-coded by L_bol) visualizes the prevalence of “normal” BHs and the selection-bias contrast.
• Figure 2: Use the reconstructed SFHs (top) to see the buildup then decline of star formation, and the back-traced M_BH offset to z≈6 (bottom) to gauge the limited evolution and compare directly to simulation/SAM prediction bands.
• Extended Fig. 2: Check the Hα+[N II]+[S II] spectral decompositions used to classify BLR emission (FWHM>1000 km/s) versus outflow-broadened components; this validates the virial mass inputs line-by-line.
• Extended Fig. 1: Prism spectra showing Balmer/4000 Å breaks, stellar absorption, and red UV–optical slopes; use the AGN flux fractions to confirm host-dominated continua and the quenching interpretation.