A Novel Formation Channel for Supermassive Black Hole Binaries in the Early Universe via Primordial Black Holes

Saiyang Zhang, Boyuan Liu, Volker Bromm

First listed 2025-08-01 | Last updated 2025-09-09

Abstract

We present a novel formation channel for supermassive black hole (SMBH) binaries in the early Universe, driven by primordial black holes (PBHs). Using high-resolution hydrodynamical simulations, we explore the role of massive PBHs ($m_{BH} \sim 10^6 M_\odot$) in catalyzing the formation of direct-collapse black holes (DCBHs), providing a natural in situ pathway for binary SMBH formation. PBHs enhance local overdensities, accelerate structure formation, and exert thermal feedback on the surrounding medium via accretion. Lyman-Werner (LW) radiation from accreting PBHs suppresses H$2$ cooling, shifting the dominant gas coolant to atomic hydrogen. When combined with significant baryon-dark matter streaming velocities ($v_{bχ} \gtrsim 0.8 σ_{bχ}$, where $σ_{bχ}$ is the root-mean-square streaming velocity), these effects facilitate the formation of dense, gravitationally unstable, atomically cooling gas clouds in the PBH's wake. These clouds exhibit sustained high inflow rates ($\dot{M}_{infall} \gtrsim 0.01 - 0.1 M_\odot yr^{-1}$), providing ideal conditions for DCBH formation from rapidly growing supermassive stars of $\sim 10^5 M_\odot$ at redshifts $z \sim 20 - 10$. The resulting systems form SMBH binaries with initial mass ratios $q \sim O(0.1)$ and separations of $\sim 10$ pc. Such PBH-DCBH binaries provide testable predictions for JWST and ALMA, potentially explaining select high-$z$ sources such as the Little Red Dots (LRDs), and represent gravitational-wave sources for future missions like LISA and TianQin-bridging early-Universe black hole physics, multi-messenger astronomy, and dark matter theory.

Short digest

High-resolution hydrodynamics around ∼10^6 M⊙ primordial black holes show that accretion-driven LW fields plus strong baryon–DM streaming (v_bχ ≳ 0.8 σ_bχ) push gas onto an atomic-cooling track and trigger collapse in the PBH wake. The collapsing clouds sustain inflow rates ≳0.01–0.1 M⊙ yr⁻¹, enabling ∼10^5 M⊙ supermassive stars that rapidly form direct-collapse BHs at z ∼ 20–10. These secondary DCBHs pair with the primary PBH to yield in-situ SMBH binaries with initial q ≈ O(0.1) at ∼10 pc separations. The channel predicts compact, LRD-like sources and low-frequency GW emitters accessible to JWST/ALMA and LISA/TianQin.

Key figures to inspect

• Figure 1: Map the LW intensity vs. Eddington ratio for different BH masses to see where the flow transitions from ADAF to thin-disk (Eq. 4 scalings) and whether the LW level required to quench H2 is plausibly reached at the fiducial radius used in the simulations.
• Figure 2: Inspect the z of first-collapse in PBH_LW_str_fd005 (ε_th = 0.005, v_bχ = 0.8 σ_bχ) to see the spatial offset between the PBH (black dot) and the collapsing cloud (blue star) and the coexistence of inflow and PBH-driven outflows that set up the wake-triggered collapse.
• Figure 3: Compare T–n_H phase diagrams across CDM, PBH_fd005, PBH_LW_fd005, and PBH_LW_str_fd005 to verify that only when both LW feedback and streaming are included does the gas track the near-isothermal atomic-cooling branch toward runaway collapse.
• Figure 4: Follow the time evolution of Ṁ_infall and collapsing-cloud mass for different streaming amplitudes to confirm sustained Ṁ ≳ 0.01–0.1 M⊙ yr⁻¹ and growth toward ∼10^5 M⊙ seeds, and to see how stronger streaming sharpens the conditions for DCBH formation relative to the primary ∼10^6 M⊙ PBH.