Binary black holes and tori in AGN

II. Can stellar winds constitute a dusty torus?

Max-Planck Institut für Radiostronomie (MPIfR), Bonn, Auf dem Hügel 69, 53121 Bonn, Germany

Corresponding author: C. Zier, chzier@mpifr-bonn.mpg.de

Received: 25 March 2002
Accepted: 12 September 2002

Abstract

In this second paper, in a series of two, we determine the properties of the stellar torus that we showed in the first 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 the binary's angular momentum. After the black holes coalesced on scales of ∼ , a geometrically thick torus remained. 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. This patchy structured torus is then compared with observations. We find the parameters of such a torus to be just in the right range in order to explain the observed large column densities in AGN and their temporal variations on time scales of about a decade. Within this model the broad absorption line quasars can be interpreted as quasars seen at intermediate inclination angles, with the line of sight grazing the edge of the torus. The half-opening angle of the torus is wider for major mergers and thus correlates with the central luminosity, as has been suggested previously. 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.