  Existing radio images of a few X-shaped radio galaxies reveal Z-symmetric
morphologies in their weaker secondary lobes which cannot be naturally
explained by either the galactic merger or radio-lobe backflow scenarios, the
two dominant models for these X-shaped radio sources. We show that the merger
picture can explain these morphologies provided one takes into account that,
prior to the coalescence of their supermassive black holes, the smaller galaxy
releases significant amounts of gas into the ISM of the dominant active galaxy.
This rotating gas, whose angular momentum axis will typically not be aligned
with the original jets, is likely to provide sufficient ram pressure at a
distance ~10 kpc from the nucleus to bend the extant jets emerging from the
central engine, thus producing a Z-symmetry in the pair of radio lobes. Once
the two black holes have coalesced some 10^7 yr later, a rapid reorientation of
the jets along a direction close to that of the orbital angular momentum of the
swallowed galaxy relative to the primary galaxy would create the younger
primary lobes of the X-shaped radio galaxy. This picture naturally explains why
such sources typically have powers close to the FR I/II break. We suggest that
purely Z-symmetric radio sources are often en route to coalescence and the
concomitant emission of substantial gravitational radiation, while X-shaped
ones have already merged and radiated.
