Dark-to-black super accretion as a mechanism for early supermassive black hole growth

Abstract: The discovery of supermassive black holes with masses $\gtrsim 10^9 M_\odot$ at redshifts $z\gtrsim 10$ challenges conventional formation scenarios based on baryonic accretion and mergers within the first few hundred million years. We propose an alternative channel in which ultralight scalar dark matter undergoes dark-to-black conversion via quasi-bound state depletion around black hole seeds. We estimate the accretion rate of the scalar field as a function of the boson mass parameter $\mu$ and the black hole mass $M_{\rm BH}$, and integrate this rate over cosmological timescales. Our results show that once a critical value of $\mu M_{\rm BH}$ is reached, scalar field accretion becomes highly efficient, enabling substantial black hole growth even from relatively small initial seed masses. For boson masses $\mu \sim 10^{-19}-10^{-16}\,\mathrm{eV}$, black hole seeds of $10^2-10^5 M_\odot$ can reach $10^6-10^8 M_\odot$ within $\sim 10^8$ yr. This dark-to-black mechanism provides a natural pathway for the rapid formation of massive black holes in the early universe, offering a potential probe of the microphysical nature of dark matter.