Exploring a primordial solution for early black holes detected with the JWST

Abstract

The James Webb Space Telescope (JWST) has unearthed black holes as massive as $10^{6.2-8.1}M_\odot$ at redshifts of $z \sim 8.5-10.6$ with many systems showing unexpectedly high black hole to stellar mass ratios >=30%, posing a crucial challenge for theoretical models. Using analytic calculations, we explore the combination of {\it astrophysical} seeding mechanisms and Eddington accretion rates that can explain the observed objects. We then appeal to {\it cosmological} primordial black hole (PBH) seeds and show how these present an alternative path for "seeding" early structures and their baryonic contents. Assuming seeding (via astrophysical means) at a redshift of $z_{\rm seed}=25$ and continuous accretion, all of the black holes studied here can either be explained through super-Eddington accretion (at an Eddington fraction of $f_{\rm Edd}<= 2.1 $) onto low-mass ($100M_\odot$) seeds or Eddington-limited accretion onto high-mass ($10^5 M_\odot$) seeds. The upper limit, where we assume a primordial origin for all of these black holes, yields a continuous primordial black hole mass function (between $10^{-5.25}$ and $10^{3.75} M_\odot$) and a fractional PBH value $<= 10^{-12}$, in good agreement with observational constraints. Starting at the redshift of matter-radiation equality ($z \sim 3400$), we show how PBH-driven structure formation can reproduce the observed stellar and black hole masses for two of the highest redshift black holes (UHZ1 and GHZ9 at $z \sim 10.3$) with the same parameters governing star formation, black hole accretion and their feedbacks. Exploring a wide swathe of model parameter space for GHZ9, we find black hole-to-stellar mass ratios ranging between $0.1-1.86$ i.e. in some cases (of high supernova feedback), the black hole grows to be more massive than the stellar mass of its host, presenting an attractive alternative to seeding these puzzling early systems.