Rotationally symmetric structure in two extragalactic radio sources

Abstract

Many extragalactic radio sources exhibit radio structures with some degree of rotational symmetry^1,2, which in extreme cases (for example, 3C315 (ref. 3) and NGC326 (ref. 4)) suggests precession of the source axis, such as might result if a relativistic beam of particles^5,6 or stream of plasmoids⁷ were to change its orientation during the source lifetime¹. In such circumstances we might expect radiative and other energy losses to steepen the spectrum of the older regions of emission⁸ away from the current sites of activity, and so make them more prominent on low-frequency maps of the source structure. The new multi-telescope radio-linked interferometer (MTRLI)⁹ at Jodrell Bank was used during January and February 1980 at a frequency of 408 MHz to map the extragalactic radio sources 3C196 and 3C305 with a resolution of ∼1 arc s. We show here that both the markedly symmetric structures observed and the spectral index distributions inferred from comparison with previously published 5-GHz maps¹⁰ provide evidence for the source axes having rotated during the lifetime of the emitting regions.