Abstract:
In pursuit of clues to the origin of
“
winged
”
or X-shaped radio galaxies
(
XRGs
)
we investigate 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 con
fi
rmed. However, several counter-examples
were also found and these could challenge the widely discussed back
fl
ow diversion model for the origin of the
radio wings. Comparison with a well-de
fi
ned 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
BH
∼
8.81
M
e
and 9.07
M
e
, respectively
)
;
(
ii
)
a much higher fraction of XRGs
(
∼
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
∼
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 dif
fi
cult to reconcile with a single dominant
physical mechanism and competing mechanisms seem prevalent.
