Orientation and size of the `Z' in X-shaped radio galaxies

Abstract

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.