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The growth efficiency of high-redshift black holes

Abstract : The observational evidence that Super-Massive Black Holes (M ∼ 109-10 M) are already in place less than 1 Gyr after the big bang poses stringent time constraints on the growth efficiency of their seeds. Among proposed possibilities, the formation of massive (∼103-6 M) seeds and/or the occurrence of super-Eddington (dot{M}>dot{M}_{Edd}) accretion episodes may contribute to the solution of this problem. In this work, using a set of astrophysically motivated initial conditions, we analytically and numerically investigate the accretion flow on to high-redshift (z ∼ 10) black holes to understand the physical requirements favouring rapid and efficient growth. Our model identifies a `feeding-dominated' accretion regime and a `feedback-limited' one, the latter being characterized by intermittent (duty cycles D ≲ 0.5) and inefficient growth, with recurring outflow episodes. We find that low-mass seeds (≲103-4 M) evolve in the feedback-limited regime, while more massive seeds (≳105-6 M) grow very rapidly as they are found in the feeding-dominated regime. In addition to the standard accretion model with a fixed matter-energy conversion factor (ɛ = 0.1), we have also explored slim disc models, appropriate for super-Eddington accretion, where radiation is trapped in the disc and the radiative efficiency is reduced (ɛ ≲ 0.04), which may ensure a continuous growth with dot{M} ≫ dot{M}_{Edd} (up to {∼ } 300 dot{M}_{Edd} in our simulations). Under these conditions, outflows play a negligible role and a black hole can accrete 80-100 per cent of the gas mass of the host halo (∼107 M) in ∼10 Myr, while in feedback-limited systems we predict that black holes can accrete only up to ∼15 per cent of the available mass.
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Fabio Pacucci, Marta Volonteri, Andrea Ferrara. The growth efficiency of high-redshift black holes. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2015, 452, pp.1922-1933. ⟨10.1093/mnras/stv1465⟩. ⟨insu-03644864⟩

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