Recoiling Supermassive Black Holes in Spin-flip Radio Galaxies
  Numerical relativity simulations predict that coalescence of supermassive
black hole (SMBH) binaries not only leads to a spin flip but also to a
recoiling of the merger remnant SMBHs. In the literature, X-shaped radio
sources are popularly suggested to be candidates for SMBH mergers with spin
flip of jet-ejecting SMBHs. Here we investigate the spectral and spatial
observational signatures of the recoiling SMBHs in radio sources undergoing
black hole spin flip. Our results show that SMBHs in most spin-flip radio
sources have mass ratio $q\ga 0.3$ with a minimum possible value $q_{\rm min}
\simeq 0.05$. For major mergers, the remnant SMBHs can get a kick velocity as
high as $2100 km s^{-1}$ in the direction within an angle $\la 40^\circ$
relative to the spin axes of remnant SMBHs, implying that recoiling quasars are
biased to be with high Doppler-shifted broad emission lines while recoiling
radio galaxies are biased to large apparent spatial off-center displacements.
We also calculate the distribution functions of line-of-sight velocity and
apparent spatial off-center for spin-flip radio sources with different apparent
jet reorientation angles. Our results show that the larger the apparent jet
reorientation angle is, the larger the Doppler-shifting recoiling velocity and
apparent spatial off-center displacement will be. We investigate the effects of
recoiling velocity on the dust torus in spin-flip radio sources and suggest
that recoiling of SMBHs would lead to "dust poor" AGNs. Finally, we collect a
sample of 19 X-shaped radio objects and for each object give the probability of
detecting the predicted signatures of recoiling SMBH.
