Double-double radio galaxies

We have discovered four radio sources whose radio morphology resembles that of two unequally sized FR-II type radio sources aligned along the same axis and with it coinciding radio cole, We present low and high resolution radio images of these four 'double-double radio galaxies' (DDRGs) from the NVSS, WENSS and FIRST radio surveys, and from our own VLA observations, together with redshifts from optical spectroscopy. We note that, using our definition, thew known radio sources 3C 445, 4C 26.35 and 1545-321 also are DDRGs. The two-sidednesss and symmetry of the inner sources strongly suggests that DDRGs are formed as the result of recurrent radio-activity in the nuclens. We discuss this in some detail and show that this leads to an evolutionary scenario in which very large radio sources can he seemingly replaced by smaller radio sources. We call this the 'metamorphosis model' of double-lobed radio sources.