The properties of primordially-seeded black holes and their hosts in the first billion years: implications for JWST

Aims. James Webb Space Telescope (JWST) observations have opened a tantalising new window onto possible black holes as early as redshifts of z ∼ 10.4. Using local relations for calibration, these systems show a number of puzzling properties including black holes as massive as MBH ≳ 108 M⊙ in place at z ∼ 10, unexpectedly high black hole-to-stellar mass ratios of MBH/M* ≳ 0.1 and, for some of them, extremely low metallicities. These pose a serious challenge for “astrophysical” seeding and growth models that we aim to explain with “cosmological” primordial black holes (PBHs) in this work. Methods. We present PHANES (Primordial black holes accelerating the assembly of nascent early structures), an analytic framework that follows the evolution of dark matter halos, and their baryons in the first billion years, seeded by a population of PBHs with seed masses between 100.5 − 106 M⊙. In addition to a fiducial model where black holes are considered non-spinning (s = 0), we also explore two “maximal” scenarios where black holes show maximally prograde (s = +1) or retrograde (s = −1) spins. Results. PBH seeded models yield a black hole mass function that extends between 101.25−11.25 (100.75−7.25) M⊙ at z ∼ 5(15) for the different models considered in this work. Interestingly, PBH-seeded models (with s = 0 or −1) naturally result in extremely high values of MBH/M* ≳ 0.25 at z ∼ 5 − 15. For a typical stellar mass of M∗ = 109 M⊙, we find an average value of MBH/M* ∼ 0.4(1.6) for s = 0(−1) at z = 5, providing a smoking gun for PBH-seeded models. Showing Eddington accretion fractions that range over two orders of magnitude (fEdd ∼ 0.01 − 1), another particularity of PBH-seeded models is their ability of producing systems with high black hole-to-stellar mass ratios that are extremely metal poor (Z ≲ 10−2 Z⊙). Yielding a PBH-to-dark matter fraction ≲10−9 and a stellar mass function that lies four orders of magnitude below observations, our model is in accord with all current cosmological and astrophysical bounds.