A possible pathway to UHZ1-type systems at z~10 by heterogeneous mass primordial black holes as dark matter

Alexander Kashlinsky, Fernando Atrio-Barandela, Diego Martín-González

First listed 2026-01-13 | Last updated 2026-01-13

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 $\sim 10^8M_\odot$ in stars, with a similarly massive central black hole of $\sim 10^{7-8}M_\odot$. 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 halos 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 halos, 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 center 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 JWST at high redshifts, including the Little Red Dots.

Short digest

Proposes a PBH–dark-matter pathway to assemble UHZ1-like systems at z~10 via heterogeneous-mass PBHs that boost small-scale power and seed earlier halo collapse. Derives an accurate expression for the PBH “granulation” contribution to the matter power, then follows 2-body relaxation and dynamical friction in the virialized halo to mass-segregate PBHs. The most massive PBHs sink to the center and can build a ~10^7–10^8 Msun black hole by z~10 while baryons form ~10^8 Msun in stars, matching UHZ1’s inferred ratios. Authors note this mechanism could explain a subset of Little Red Dots, contingent on a suitable PBH mass function contributing a significant fraction of DM.

Key figures to inspect

• Figure 1: Inspect how the PBH granulation term elevates small-scale power over CDM and how the analytic fit (Eq. 2) tracks the numerical spectrum at the k relevant for first-halo collapse; note the collapse-threshold lines.
• Figure 2: Read off which halo masses reach δ/δ_c sufficient to collapse by the marked redshifts with and without PBHs; compare against the vertical T_vir=10^3/10^4 K lines and the shaded UHZ1 BH and stellar mass bands to see where the model lands.
• Figure 3: Use t_df/t_cross versus PBH-to-halo mass fraction and concentration to identify when massive PBHs sink before the halo’s survival time from high z to z~10; contrast NFW vs isothermal cases for sensitivity to c.
• Figure 4: Check how advection modifies the abundance of collapsed halos relative to the no-advection case across f_PBH; focus on changes near the cooling-threshold masses that regulate when gas can form stars.