A possible pathway to UHZ1-type systems at <i>z</i> ∼ 10 by heterogeneous mass primordial black holes as dark matter

ABSTRACT Recent space-based observations discovered several unusual objects, exhibiting similar properties, at redshifts $z\gtrsim 10$. Among them is the UHZ1 system at $z=10.1$, containing $\sim 10^8\,\mathrm{ M}_\odot$ in stars, with a similarly massive central black hole of $\sim 10^{7-8}\,\mathrm{ M}_\odot$. Here we propose a possible mechanism for forming such systems which hinges on the presence of primordial black holes (PBHs) covering a range of masses while contributing a significant fraction of the dark matter (DM). We evaluate the accurate expression for the small-scale power responsible for the collapse of the first haloes in the presence of the PBH population. The extra power in the matter density field, produced by the granulation term, will cause an earlier collapse of DM haloes, populated by PBHs of different masses. In these collapsed and virialized systems the PBHs will undergo 2-body relaxation, driving the more massive PBHs to the halo centre under dynamical friction. We quantify this evolution for a distribution of PBH orbital parameters and halo properties. The analysis shows that PBHs can have appropriate mass functions capable of producing systems with parameters similar to what is observed for UHZ1. We suggest that the proposed mechanism could account for a subset of other systems newly discovered with the James Webbn Space Telescope (JWST) at high redshifts, including the Little Red Dots.