Growing Supermassive Black Holes by Chaotic Accretion

Abstract

We consider the problem of growing the largest supermassive black holes from stellar--mass seeds at high redshift. Rapid growth without violating the Eddington limit requires that most mass is gained while the hole has a low spin and thus a low radiative accretion efficiency. If, as was formerly thought, the black--hole spin aligns very rapidly with the accretion flow, even a randomly--oriented sequence of accretion events would all spin up the hole and prevent rapid mass growth. However, using a recent result that the Bardeen--Petterson effect causes {\it counter}alignment of hole and disc spins under certain conditions, we show that holes can grow rapidly in mass if they acquire most of it in a sequence of randomly oriented accretion episodes whose angular momenta $J_d$ are no larger than the hole's angular momentum $J_h$. Ultimately the hole has total angular momentum comparable with the last accretion episode. This points to a picture in which the accretion is chaotic on a lengthscale of order the disc size, that is $\la 0.1$ pc.