Seeding mechanism of massive black holes at high redshift

Determine the seeding mechanism(s) of massive black holes at high redshift that can explain the presence of ~10^7–10^8 solar-mass black holes by redshift z≈4–6 in compact galaxies identified as Little Red Dots, consistent with current JWST and related observations.

Background

Observations with JWST reveal a population of compact, high-redshift systems (Little Red Dots) that likely host massive black holes with masses up to 107–108 solar masses by z~4–6, and possibly as early as z~10. Multiple theoretical pathways have been proposed for forming the initial seeds, including runaway stellar collisions in dense clusters, direct collapse of gas, or other channels, but their relative contributions remain uncertain.

The paper develops a semi-analytical framework in which nuclear star clusters evolve and can grow central black holes through tidal disruption events, captures of stellar-mass black holes, and gas accretion. Despite modeling growth from plausible initial conditions, identifying and constraining the initial seeding channel(s) is explicitly noted as unresolved.

References

We are not currently in a position to fully constrain the seeding mechanism of massive BHs at high redshift.

— From nuclear star clusters to Little Red Dots: black hole growth, mergers, and tidal disruptions  (2510.21709 - Kritos et al., 24 Oct 2025) in Section 1: Introduction

Several issues remain open. Most importantly, the present analysis does not derive the required alignment and multi-harmonic structure from a specific heterotic compactification. Establishing such a construction would provide a microscopic test of the scenario. Likewise, the $10{-28}$ eV sector should ultimately be confronted with the full set of CMB, BAO, supernova, and structure-formation constraints through a numerical cosmological analysis. Such an analysis is necessary to determine quantitatively how large a reduction of the sound horizon can be obtained while remaining consistent with the other cosmological observables. On the $10{-22}$ eV side, a quantitative treatment of the nonlinear condensation--collapse--recondensation cycle will be necessary to determine the resulting seed-mass distribution and to test its possible relation to the observed high-redshift compact-object population.

— Toward a Unified Axion Cosmology  (2608.22667 - Fukuyama, 24 Aug 2026) in Section 4, Discussion and conclusions