Rapid Dark Growth of Seed Black Holes in Self-Interacting Dark Matter

Yu Rong, Shuang-Nan Zhang, Jian-Min Wang, Junxian Wang

First listed 2026-09-18 | Last updated 2026-09-17

Abstract

The rapid emergence of massive black holes in the early Universe, possibly related to Little Red Dots, challenges conventional growth from light seeds. Dark-matter accretion offers a distinct route, and self-interacting dark matter (SIDM) makes it possible by behaving as a collisional fluid. We show that gas inflow can drive strong local contraction of an SIDM halo, assembling a dense reservoir around a pre-existing black-hole seed. The seed then enters a rapid dark-growth phase and can gain several orders of magnitude in mass within less than $1$ Myr before transitioning to slower accretion. We self-consistently follow this process by solving the spherical Euler equations with SIDM self-gravity, collisional heat transport, and an absorbing black-hole sink, rather than imposing an accretion history. The growth depends most strongly on halo mass, with weaker sensitivity to inflowing gas fraction, nuclear size, and seed mass. Gas-inflow-driven SIDM accretion therefore provides an efficient pathway for producing massive black holes in the early Universe.

Short digest

This paper models a distinct early-black-hole growth channel in which merger- or compaction-driven gas inflow contracts a self-interacting dark-matter halo and builds a dense SIDM reservoir around a pre-existing seed. Solving time-dependent spherical SIDM fluid equations with self-gravity, conductive heat transport, and an absorbing black-hole sink, the authors find a brief collisional dark-accretion burst that can add several orders of magnitude in mass in under 1 Myr before the flow loses efficient collisional coupling. Halo mass is the dominant control on the effective saturation mass, whereas gas fraction, nuclear size, and seed mass have weaker effects, making baryon-compressed SIDM a potentially efficient route to the massive seeds implicated by little red dots.

Key figures to inspect

• Figure 1. This is the paper’s central result figure: it follows the numerical black-hole mass growth during gaseous-nucleus assembly, identifies the effective collisional endpoint where the SIDM flow becomes less efficiently coupled, and contrasts the finite-reservoir calculation with the ideal unlimited-supply model. It makes clear that the rapid burst is substantial but terminates well below the formal ideal-growth outcome, while showing that halo mass drives the strongest variation across the parameter grid.