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Both the X-shaped radio galaxies and double-double radio galaxies (DDRGs) are suggested in the literature to be due to the binary-accretion disk interaction or to the coalescence of SMBBHs. These models suggest some relationship between the two types of radio sources. In this paper, we collected data from literatures for two samples of X-shaped and double-double radio galaxies together with a control sample of FRII radio galaxies and statistically investigate their properties.
We find that the wings of X-shaped radio galaxies and the outer and inner lobes of DDRGs tend to be perpendicular to the major axis of the host galaxy (or dust structures), while the active lobes orient randomly. Both X-shaped and double-double radio galaxies are low luminous FRII or FRI/FRII transitional radio sources with the similar dimensionless accretion rate ṁ ∼ 0.01, which is about the transitional accretion rate given in the literature.
All the statistic results can be reconciled if there is an evolutionary relationship between X-shaped and double-double radio galaxies, in the sense that X-shaped radio galaxies may be due to the interaction of active SMBBHs and accretion disk and DDRGs due to the removal of inner disk region and the coalescence of SMBBHs.