Double-double radio galaxies: remnants of merged supermassive binary black holes

The activity of active galaxies may be triggered by the merging of galaxies, and present-day galaxies are probably the product of successive minor mergers. The frequent galactic mergers at high redshift imply that active galaxies harbour supermassive unequal-mass binary black holes at their centre at least once during their lifetime. The secondary black hole interacts and becomes coplanar with the accretion disc around the primary, inwardly spiraling toward their mass centre owing to the loss of orbital angular momentum to the disc mass outside the orbit of the secondary and/or to gravitational radiation. The binary black holes finally merge and form a more massive (post-merged) black hole at the centre. In this paper it is shown that the recently-discovered double-lobed FR II radio galaxies are the remnants of such supermassive binary black holes. The inwardly spiraling secondary black hole opens a gap in the accretion disc that increases with time when the loss of the orbital angular momentum via gravitational radiation becomes dominant. When the supermassive black holes merge, the inner accretion disc disappears and the gap becomes a big hole of several hundreds of Schwarzschild radii in the vicinity of the post-merged supermassive black hole, leading to an interruption of jet formation. When the outer accretion disc slowly refills the big hole on a viscous time-scale, jet formation restarts and the interaction of the recurrent jets and the inter-galactic medium forms a secondary pair of lobes. The model is applied to a particular double-lobed radio source-B1834+620-which has an interruption time-scale similar to1 Myr. It is shown that the orbit of the secondary in B1834+620 is elliptical with a typical eccentricity e similar or equal to 0.68 and that the ratio q of the mass of the secondary to that of the primary is 0.01less than or similar toqless than or similar to0.4. The accretion disc is a standard alpha-disc with 0.01less than or similar toalphaless than or similar to0.04 and the ratio of disc half height H to radius r is deltasimilar or equal to0.01. The model predicts that double-lobed radio structures form only in FR II or borderline FR I/FR II radio galaxies and that the detection rate of double-lobed radio sources among FR II radio sources is about one per cent.