On the origin of X-shaped radio galaxies
  After a brief, critical review of the leading explanations proposed for the
small but important subset of radio galaxies showing an X-shaped morphology
(XRGs) we propose a generalized model, based on the jet-shell interaction and
spin-flip hypotheses. The most popular scenarios for this intriguing phenomenon
invoke either hydrodynamical backflows and over-pressured cocoons or rapid jet
reorientations, presumably from the spin-flips of central engines following the
mergers of pairs of galaxies, each of which contains a supermassive black hole
(SMBH). We confront these models with a number of key observations and thus
argue that none of the models is capable of explaining the entire range of
salient observational properties of XRGs, although some of the arguments raised
in the literature against the spin-flip scenario are probably not tenable. We
then propose here a new scenario which also involves galactic mergers but would
allow the spin of the central engine to maintain its direction. Motivated by
the detailed multi-band observations of the nearest radio galaxy, Centaurus A,
this new model emphasizes the role of interactions between the jets and the
shells of stars and gas that form and rotate around the merged galaxy and can
cause temporary deflections of the jets, occasionally giving rise to an
X-shaped radio structure. Although each of the models is likely to be relevant
to a subset of XRGs, the bulk of the evidence indicates that most of them are
best explained by the jet-shell interaction or spin-flip hypotheses.
