Dark-to-black super-accretion as a spin-imprinting mechanism for supermassive Kerr black holes
The existence of supermassive black holes with masses $M\gtrsim10^9\,M_{\odot}$ and large dimensionless spins $χ\sim0.9-0.99$ at high redshift remains a challenge to our understanding of the early Universe. In this work, we study the adiabatic co-evolution of a Kerr black hole seed surrounded by two ultralight scalar dark matter clouds occupying different bound states, and show that this configuration allows the black hole to grow into the supermassive mass range while imprinting a characteristic final spin. The evolution proceeds through two stages. During the first stage, a spherical cloud described by the $\ell=0$ mode is completely depleted through a runaway dark-to-black accretion mechanism on a timescale of hundreds of millions of years for boson masses $μ\sim10^{-18}-10^{-17}\,\mathrm{eV}$. Since the accreted material does not carry angular momentum, the black hole spin is universally driven to $χ\simeq0$, independently of its initial spin. Throughout this stage, the second cloud, described by the $\ell=m=1$ mode, remains in the superradiant regime with negligible evolution. However, once the first stage is completed, this cloud transitions to the accreting regime, rapidly transferring both mass and angular momentum to the black hole. Starting from $χ\simeq0$, the black hole spin increases until the evolution self-consistently saturates close to the threshold $χ_{\rm sat}$, defined by the condition $Ω_H(χ_{\rm sat})=μ$, on an e-folding timescale of thousands of years, orders of magnitude shorter than the first stage. This final saturation spin is largely independent of both the initial black hole spin and the mass of the secondary cloud, providing a spin-imprinting mechanism in which the primordial spin is first erased by spherical accretion and then reset to a value determined only by the boson mass and the final black hole mass.