Size and Orientation of the `Z' in ZRGs

Some X-shaped radio galaxies (XRGs) show a Z-symmetric morphology in the less luminous secondary lobes. Our geometrical arguments strongly support a merger of two galaxies as mechanism for the formation of these sources (ZRG). They also strengthen the conjecture that a jet is aligned with the spin of the BH at its base and that the jet flips into the direction of the orbital angular momentum of the pre-merger binary black hole (BHB). We could also restrict the distance where the pre-merger jet is bent into Z-shape by the inspiralling galaxy to the range of 30-100 kpc. One of three possible orientations of the jet relative to our line of sight is more likely than the others and allows us to deduce the direction of the spin of the merged BH. The existence of XRGs and ZRGs proves that the binary has merged, contrary to previous speculations that after a merger of two galaxies the decay of the BHB stalls due to loss cone depletion. In ZRGs the black holes probably merge on timescales of some 10^8 yr after the bending of the jet in a distance of about 50 kpc. Thus, in a way, the bending starts a stop watch for the rest of the merger.