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

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.