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

Abstract

James Webb Space Telescope (JWST) observations have opened a tantalising new window onto possible black holes as early as redshifts of $z \sim 10.4$. These show a number of puzzling properties including unexpectedly massive black holes in place by $z \sim 10$ and inexplicably high black hole-to-stellar mass ratios of $M_{\rm BH}/M_*\geq 0.1$. 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. We present PHANES, 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 $10^{0.5}-10^6 M_\odot$. PBH seeded models yield a black hole mass function that extends between $10^{1.25-11.25} ~(10^{0.75-7.25})M_\odot$ at $z \sim 5 (15)$ for the different models considered in this work. Interestingly, PBH-seeded models (with spin $s=0$ or $-1$) naturally result in extremely high values of $M_{\rm BH}/M_*\geq 0.25$ at $z \sim 5-15$. For a typical stellar mass of $M_* =10^9 M_\odot$, we find an average value of $M_{\rm BH}/M_* \sim 0.4~ (1.6)$ for $s=0~(-1)$ at $z=5$, providing a smoking gun for PBH-seeded models. 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 ($Z \leq 10^{-2}~Z_\odot$). Yielding a PBH-to-dark matter fraction $\leq 10^{-9}$ 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.