Chaotic accretion can (still) explain early supermassive black hole growth

The origin of supermassive black holes is still debated, with proposed scenarios including massive seeds, super-Eddington growth, and chaotic accretion. Recent discoveries of very high-redshift supermassive black holes constrain their possible origin and growth, and this work investigates whether chaotic accretion can explain the most extreme high-redshift black hole masses.

The authors predicted the exponential growth timescale for black holes accreting chaotically in a slowly-spinning dark matter halo, where uncorrelated gas inflows maintain low spin and therefore low radiative efficiency. They compared this prediction against the growth timescales and seed masses implied by a compilation of all known supermassive black hole mass estimates at z>7, and against the feeding and feedback conditions expected in such haloes.

They found that in the period 7.6 < z < 10.6, the most massive supermassive black holes likely grew very efficiently, with an exponential growth timescale of tau ~32 Myr. This is consistent with chaotic accretion predictions provided that growth was almost continuous. All high-redshift supermassive black holes with virial mass estimates could have grown from seeds with M_seed <~ 100 solar masses.

The paper also describes the evolution of the mass reservoir feeding such a black hole, suggesting that wind-driven feedback eventually removes most of the gas and changes the growth mode of black holes at z ~ 7.5. It makes several predictions regarding spins, host-galaxy correlations, and observable counterparts of high-redshift supermassive black holes that distinguish this model from alternatives, and these predictions will be testable in the near future. The conclusion is that chaotic accretion from stellar-mass seeds remains a viable origin for the earliest known supermassive black holes, without invoking massive seeds or super-Eddington growth.