Binary black holes and tori in AGN II. Can stellar winds constitute a dusty torus?

TL;DR

This paper proposes a model where stellar winds from stars in a binary black hole merger remnant form a dusty, optically thick torus in AGN, explaining observed properties and variability, and linking to phenomena like BAL quasars and X-shaped radio galaxies.

Contribution

It introduces a new model for the AGN dusty torus formed by stellar winds shaped by radiation pressure, connecting black hole mergers to observed AGN features and jet reorientations.

Findings

The stellar winds can produce an optically thick, patchy torus with observed column densities.

The model explains the variability of AGN column densities over about a decade.

It suggests a link between black hole merger dynamics and AGN observational phenomena.

Abstract

We determine the properties of the stellar torus that we showed in a previous paper to result as a product of two merging black holes. If the surrounding stellar cluster is as massive as the binary black hole, the torque acting on the stars ejects a fraction which extracts all the binary's angular momentum on scales of ~10^7 yr, and a geometrically thick torus remains. In the present article we show that a certain fraction of the stars has winds, shaped into elongated tails by the central radiation pressure, which are optically thick for line of sights aligned with them. These stars are sufficiently numerous to achieve a covering factor of 1, so that the complete torus is optically thick. We find the parameters of such a patchy torus to be in the right range to explain the observed large column densities in AGN and their temporal variations on time scales of about a decade. Within this model the BAL quasars can be interpreted as quasars seen at intermediate inclination angles, with the line of sight grazing the edge of the torus. The opening angle of the torus is wider for major mergers and thus correlates with the central luminosity. In this picture the spin of the merged black hole is possibly dominated by the orbital angular momentum of the binary. Thus the spin of the merged black hole points into a new direction, and consequently the jet experiences a spin-flip according to the spin-paradigm. This re-orientation could be an explanation for the X-shaped radio galaxies, and the advancing of a new jet through the ambient medium for Compact Symmetric Objects.