Abstract.Observations of HST and groundbased data strongly suggest that most galaxies harbour central supermassive black holes and that most galaxies merge with others. Consequently a black hole binary emerges as the two black holes are spiraling into the center towards each other. In our work we are investigating two basic questions of our understanding of the central activity of galaxies and find that both can be answered with "yes": (1) Do the black holes actually merge? (2) And does the effect of the torque of the black hole binary on the surrounding stellar distribution help to explain the presence of the ubiquitous torus of molecular material surrounding apparently all active galactic nuclei? The first question is the topic of the present article, while the second question will be subject of a subsequent paper. Simulating the evolution of a stellar cluster in the potential of such a binary by solving the equations of the restricted three body problem we obtained the following results: provided that the cluster is about as massive as the black hole binary the two black holes coalesce after ∼10 7 yr due to ejection of stars and finally via emission of gravitational radiation. Whether a star is ejected or not crucially depends on its angular momentum. Almost all stars whose angular momentum is twice as large as that of a star circulating around the binary in a distance corresponding to that between the black holes, stay bound to the binary. In a sequence of models where the mass of the secondary black hole increases while M1 is fixed, a bigger fraction of stars is ejected. For a more massive binary also the cluster has to be more massive in order to allow the two black holes to coalesce. The merger then proceeds on smaller time scales. The cluster is depleted in the central region and the final distribution of stars assumes a torus-like structure, peaking at three times the initial distance of the two black holes. The relationship of the bound stars to the obscuring torus in active galactic nuclei will be investigated in a subsequent paper.

Some X-shaped radio galaxies show a Z-symmetric morphology in the less luminous secondary lobes. Within the scenario of a merger between two galaxies, each hosting a supermassive black hole in its centre, this structure has been explained before. As the smaller galaxy spirals towards the common centre, it releases gas to the interstellar medium of the larger active galaxy. The ram pressure of this streaming gas will bend the lobes of the pre-merger jet into a Z-shape. After the black holes have merged, the jet propagates in a new direction that is aligned with the angular momentum of the binary black hole. In this paper we deproject the pre-and post-merger jets. Taking into account the expected angles between the jet pairs and with the assumption that their directions are uncorrelated, we show that one of three possible orientations of the jets with respect to the line of sight is more likely than the others. This actually depends on the distance where the bending occurs. Another result of our deprojection is that the streaming gas bends the jet into a Z-shape in a range between about 30 and 100 kpc distance to the centre of the primary galaxy. We confirm this finding by comparing our predictions for the properties of the rotational velocity field and its radius with observations and numerical simulations of merging galaxies. Thus, our results support the merger scenario as explanation for X-and Z-shaped radio galaxies with the jet pointing along the former axis of orbital angular momentum of the binary.

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 ∼10 7 yr, 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.

The origin of ultra high energy cosmic rays promises to lead us to a deeper understanding of the structure of matter. This is possible through the study of particle collisions at center-of-mass energies in interactions far larger than anything possible with the Large Hadron Collider, albeit at the substantial cost of no control over the sources and interaction sites. For the extreme energies we have to identify and understand the sources first, before trying to use them as physics laboratories. Here we describe the current stage of this exploration. The most promising contenders as sources are radio galaxies and gamma ray bursts. The sky distribution of observed events yields a hint favoring radio galaxies. Key in this quest are the intergalactic and galactic magnetic fields, whose strength and structure are not yet fully understood. Current data and statistics do not yet allow a final judgement. We outline how we may progress in the near future.

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