2024A&A...690A.182D

Aims. With its rest-frame optical sensitivity, the James Webb Space Telescope (JWST) has unearthed black holes as massive as 10^6.2–8.1 M_⊙ at redshifts of z ∼ 8.5 - 10.6. In addition to these unexpectedly high masses, many systems show unexpectedly high ratios of the black hole to stellar mass of M_BH/M_* ≳ 30% at these early epochs. This poses a crucial challenge for theoretical models.
Methods. We collated data for all of the black holes that were confirmed with the JWST (through spectroscopy, X-rays, or high-ionization emission lines). Using analytic calculations, we explored the combination of astrophysical seeding mechanisms and Eddington accretion rates that can explain the observed objects. We then appeal to cosmological primordial black hole (PBH) seeds and show that these present an alternative path for the seeding of early structures and their baryonic contents.
Results. When we assume seeding (via astrophysical means) at a redshift of z_seed = 25 and continuous accretion, all of the black holes we studied can analytically either be explained through super-Eddington accretion (at an Eddington fraction of f_Edd ≲ 2.1) onto low-mass seeds (100 M_⊙) or through Eddington-limited accretion onto high-mass seeds (10⁵ M_⊙). The upper limit at which we assume a primordial origin for all of these black holes yields a continuous primordial black hole mass function (between 10^–5.25 and 10^3.75 M_⊙) and a fractional PBH value ≲10^–12. This agrees well with observational constraints. We then show that PBHs can seed a halo around themselves and assemble their baryonic (gas and stellar contents) starting at the redshift of matter-radiation equality (z ∼ 3400). We were able to reproduce the observed stellar and black hole masses for two of the highest-redshift black holes (UHZ1 and GHZ9 at z ∼ 10.3) with the same parameters as those that govern star formation, black hole accretion, and their feedbacks. Exploring a wide swathe of model parameter space for GHZ9, we find ratios of black hole to stellar mass between 0.1 - 1.86. This means that in some cases (of high supernova feedback), the black hole grows to be more massive than the stellar mass of its host halo. This is an attractive alternative to seeding these puzzling early systems.