Bondi flow from a slowly rotating hot atmosphere

TL;DR

This paper explores how viscosity-driven angular momentum transport in a hot, rotating atmosphere around a supermassive black hole leads to steady accretion rates close to the classical Bondi rate, enabling effective feedback mechanisms.

Contribution

It introduces a model of ADAF with significant viscosity and rotation, showing that accretion rates remain high despite angular momentum, unlike previous models.

Findings

Accretion rates are only a factor of a few below classical Bondi estimates.

Infall times are a few times the free-fall time, supporting rapid accretion.

Jet powers are a few percent of the accretion energy, consistent with observations.

Abstract

A supermassive black hole in the nucleus of an elliptical galaxy at the centre of a cool-core group or cluster of galaxies is immersed in hot gas. Bondi accretion should occur at a rate determined by the properties of the gas at the Bondi radius and the mass of the black hole. X-ray observations of massive nearby elliptical galaxies, including M87 in the Virgo cluster, indicate a Bondi accretion rate Mdot which roughly matches the total kinetic power of the jets, suggesting that there is a tight coupling between the jet power and the mass accretion rate. While the Bondi model considers non-rotating gas, it is likely that the external gas has some angular momentum, which previous studies have shown could decrease the accretion rate drastically. We investigate here the possibility that viscosity acts at all radii to transport angular momentum outward so that the accretion inflow proceeds rapidly and steadily. The situation corresponds to a giant Advection Dominated Accretion Flow (ADAF) which extends from beyond the Bondi radius down to the black hole. We find solutions of the ADAF equations in which the gas accretes at just a factor of a few less than Mdot. These solutions assume that the atmosphere beyond the Bondi radius rotates with a sub-Keplerian velocity and that the viscosity parameter is large, alpha~0.1. The infall time of the ADAF solutions is no more than a few times the free-fall time. Thus the accretion rate at the black hole is closely coupled to the surrounding gas, enabling tight feedback to occur. We show that jet powers of a few per cent of Mdot c^2 are expected if either a fraction of the accretion power is channeled into the jet or the black hole spin energy is tapped by a strong magnetic field pressed against the black hole by the pressure of the accretion flow.(Truncated)