X-shaped Radio Galaxies: Optical Properties, Large-scale Environment and
  Relationship to Radio Structure
  In order to find clues to the origin of the "winged" or "X-shaped" radio
galaxies (XRGs) we investigate here the parent galaxies of a large sample of
106 XRGs for optical-radio axes alignment, interstellar medium, black hole
mass, and large-scale environment. For 41 of the XRGs it was possible to
determine the optical major axis and the primary radio axis and the strong
tendency for the two axes to be fairly close is confirmed. However, several
counter-examples were also found and these could challenge the widely discussed
backflow diversion model for the origin of the radio wings. Comparison with a
well-defined large sample of normal FR II radio galaxies has revealed that: (i)
XRGs possess slightly less massive central black holes than the normal radio
galaxies (average masses being log$M_{\rm BH} \sim$ 8.81 $M_{\odot}$ and 9.07
$M_{\odot}$, respectively); (ii) a much higher fraction of XRGs ($\sim$ 80%)
exhibits red mid-IR colors ($W2 - W3 > 1.5$), indicating a population of young
stars and/or an enhanced dust mass, probably due to relatively recent galaxy
merger(s). A comparison of the large-scale environment (i.e., within $\sim$ 1
Mpc) shows that both XRGs and FRII radio galaxies inhabit similarly poor galaxy
clustering environments (medium richness being 8.94 and 11.87, respectively).
Overall, the origin of XRGs seems difficult to reconcile with a single dominant
physical mechanism and competing mechanisms seem prevalent.
