Rapid growth of black holes accompanied with hot or warm outflows exposed to anisotropic super-Eddington radiation

Abstract

We perform two-dimensional radiation hydrodynamical simulations of accretion flows onto a black hole (BH) with a mass of $10^3\le M_{\mathrm{BH}}/M_{\odot }\lesssim 10^6$ in order to study rapid growth of BHs in the early Universe. For spherically symmetric flows, hyper-Eddington accretion onto the BH from outside the Bondi radius can occur unimpeded by radiation feedback only when the BH mass is higher than $\simeq 10^4{M}_{\odot }(n_{\infty }/10^5{\mathrm{cm}}^{-3}{)}^{-1}(T_{\infty }/10^4\mathrm{K}{)}^{3/2}$, where $n_{\infty }$ and $T_{\infty }$ are the density and temperature of ambient gas. Here, we study the properties of accretion flows exposed to anisotropic radiation from a nuclear accretion disk with a luminosity higher than the Eddington value ($L_{\mathrm{Edd}}$) due to collimation toward the bipolar directions. We find that, unlike the spherically symmetric case, even less massive BHs with $M_{\mathrm{BH}}<10^4{M}_{\odot }$ can be fed by surrounding gas at high accretion rates of $\gtrsim L_{\mathrm{Edd}}/c^2$ through the equatorial plane, while ionized regions expand to the polar directions producing hot outflows with $T\sim 10^5$K. For more massive BHs with $M_{\mathrm{BH}}\gtrsim 5\times 10^5{M}_{\odot }$, neutral gas through the equatorial plane totally covers the central radiating region due to the non-radial gas motions, and thus the emergent radiation in all directions is blocked. Because of efficient recombination by hydrogen, the entire flow results in neutral and warm gas with $T\simeq 8000\mathrm{K}$ . The central BH is fed through the equator at the averaged rate of $\sim 5\times 10^4{L}_{\mathrm{Edd}}/c^2$, which corresponds to $\sim 50\%$ of the inflow rate from the Bondi radius. Moreover, radiation momentum absorbed by neutral hydrogen produces warm outflows toward the bipolar directions at $\sim 30\%$ of the BH feeding rate and with a typical velocity of $\simeq 50{\mathrm{km}\mathrm{s}}^{-1}$.

10 pages, 10 figures