2024MNRAS.531.4584S

Supermassive black holes (SMBHs) with masses of ∼10⁹ M_⊙ within the first billion year of the universe challenge our conventional understanding of black hole formation and growth. One pathway to these SMBHs proposes that supermassive stars born in pristine atomic cooling haloes yield massive seed BHs evolving to these early SMBHs. This scenario leads to an overly massive BH galaxy (OMBG), in which the BH to stellar mass ratio is initially M_bh/M_* ≥ 1, well in excess of the typical values of ∼10^–3 at low redshifts. Previously, we have investigated two massive seed BH candidates from the Renaissance simulation and found that they remain outliers on the M_bh-M_* relation until the OMBG merges with a much more massive halo at z = 8. In this work, we use Monte-Carlo merger trees to investigate the evolution of the M_bh-M_* relation for 50 000 protogalaxies hosting massive BH seeds, across 10 000 trees that merge into a 10¹² M_⊙ halo at z = 6. We find that up to 60 per cent (depending on growth parameters) of these OMBGs remain strong outliers for several 100 Myr, down to redshifts detectable with JWST and with sensitive X-ray telescopes. This represents a way to diagnose the massive-seed formation pathway for early SMBHs. We expect to find ∼0.1-1 of these objects per JWST Near Infrared Camera (NIRCam) field per unit redshift at z ≳ 6. Recently detected SMBHs with masses of ∼10⁷ M_⊙ and low-inferred stellar-mass hosts may be examples of this population.