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 M BH ≳ 10 8 M ⊙ in place at z ∼ 10, unexpectedly high black hole-to-stellar mass ratios of M BH / 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 ( P rimordial black h oles a ccelerating the assembly of n ascent e arly s tructures), 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 10 0.5 − 10 6 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 10 1.25−11.25 (10 0.75−7.25 ) 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 M BH / M * ≳ 0.25 at z ∼ 5 − 15. For a typical stellar mass of M ∗ = 10 9 M ⊙ , we find an average value of M BH / 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 ( f Edd ∼ 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.