2016MNRAS.459.3738I

We study very high rate, spherically symmetric accretion flows on to massive black holes (BHs; 10² <= M_BH <= 10⁶ M_☉) embedded in dense metal-poor clouds, performing one-dimensional radiation hydrodynamical simulations. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback when (n_[?]/10⁵/cm3) > (M_BH/10⁴ M_☉)^–1(T_[?]/10⁴ K)^3/2, where n_[?] and T_[?] are the density and temperature of ambient gas. Accretion rates in this regime are steady, and larger than 5000L_Edd/c², where L_Edd is the Eddington luminosity. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. In the hyper-Eddington case, the solution consists of a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T ≃ 8000 K. When the emergent luminosity is limited to <= L_Edd because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of T_vir >= 10⁴K. Once a seed BH forms at the centre of the galaxy, it can grow to a maximum ∼10⁵(T_vir/10⁴ K) M_☉ via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lyα emitters without X-rays and nearby luminous infrared sources with hot dust emission, respectively.