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

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 ($v_{b\chi} \gtrsim 0.8 \sigma_{b\chi}$, where $\sigma_{b\chi}$ 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.