  Formation of supermassive black holes (BHs) remains a theoretical challenge.
In many models, especially beginning from stellar relic "seeds," this requires
sustained super-Eddington accretion. While studies have shown BHs can violate
the Eddington limit on accretion disk scales given sufficient "fueling" from
larger scales, what remains unclear is whether or not BHs can actually capture
sufficient gas from their surrounding ISM. We explore this in a suite of
multi-physics high-resolution simulations of BH growth in magnetized,
star-forming dense gas complexes including dynamical stellar feedback from
radiation, stellar mass-loss, and supernovae, exploring populations of seeds
with masses $\sim 1-10^{4}\,M_{\odot}$. In this initial study, we neglect
feedback from the BHs: so this sets a strong upper limit to the accretion rates
seeds can sustain. We show that stellar feedback plays a key role. Complexes
with gravitational pressure/surface density below $\sim 10^{3}\,M_{\odot}\,{\rm
pc^{-2}}$ are disrupted with low star formation efficiencies so provide poor
environments for BH growth. But in denser cloud complexes, early stellar
feedback does not rapidly destroy the clouds but does generate strong shocks
and dense clumps, allowing $\sim 1\%$ of randomly-initialized seeds to
encounter a dense clump with low relative velocity and produce runaway,
hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass
growth under these conditions is almost independent of initial BH mass,
allowing rapid IMBH formation even for stellar-mass seeds. This defines a
necessary (but perhaps not sufficient) set of criteria for runaway BH growth:
we provide analytic estimates for the probability of runaway growth under
different ISM conditions.
