Super-Eddington Growth Ceiling: Analytic Constraints on the Rapid Growth of Light-Seed Black Holes in Massive Clumps

Abstract

The existence of $\sim10^{7-8}{\rm M}_\odot$ supermassive black holes at $z\gtrsim 8$ challenges conventional growth channels. One attractive possibility is that light seeds ($M_{\rm BH}\lesssim10^{3}{\rm M}_\odot$) undergo short, super-Eddington episodes when they cross, and are captured by, dense massive gas clumps. We revisit this ``BH-clump-capture'' model using analytic arguments supported by toy-model simulations that follow Bondi-scale inflow, radiative feedback, gas dynamical friction and the recently discovered forward acceleration effect caused by the ionised bubble. For substantial growth the black hole must remain trapped for many dynamical times, which imposes three simultaneous constraints. The clump must be heavier than the black hole (mass doubling condition); its cooling time must exceed the super-Eddington growth time (lifetime condition); and dynamical friction must dominate shell acceleration (BH-trapping condition). These requirements confine viable clumps to a narrow density-temperature region, $n_{\rm H}\simeq10^{7-8}{\rm cm}^{-3}$ and $T\simeq(2-6)\times10^{3}{\rm K}$, for a $10^{3}{\rm M}_\odot$ seed. Even inside this sweet spot a $10^{3}{\rm M}_\odot$ seed grow up at most $4\times10^{3}{\rm M}_\odot$; the maximum growth ratio $M_{\rm BH, fin}/M_{\rm BH}$ falls approximately as $M_{\rm BH}^{-0.4}$ and is negligible once $M_{\rm BH}\gtrsim10^{4}{\rm M}_\odot$. The forward-acceleration effect is essential, expelling the black hole whenever photon trapping fails. We conclude that BH-clump-capture model, and potentially broad super-Eddington models, cannot produce the $>10^{4}{\rm M}_\odot$ seeds required for subsequent Eddington-limited growth, suggesting that alternative pathways, such as heavy seed formation, remain necessary.