NGC326—a radio galaxy with a precessing beam?

Abstract

THE radio emission from most strong extragalactic sources is concentrated in a band which passes through, and is symmetrical with respect to, the associated galaxy¹. This structure is sometimes modified to a V or C shape with the galaxy at the apex, as in the wide angle tail sources like 3C465 (ref. 2), and occasionally to the more extreme elongation of the tail sources such as NGC1265 (ref. 3). In a few sources an S-shaped or 180° symmetry is seen. This was first noted in the outer components of Cen A⁴ and a better example is 3C272.1 (refs 5, 6). Other sources where the S distortion is very clear have been found with higher resolution observations (for example Cyg A⁷ and 3C47⁸). In these, the structure in question subtends a small angle from the centre and they give a strong impression of a beam type of model in which the beam has wobbled or precessed by a few degrees⁹. A new example of a radio source showing a structure with a 180° symmetry, but not of the S form, has been found as part of a systematic study of radio galaxies of lower luminosity selected from the B2 catalogue¹⁰. The projection of a precessing beam model seems to provide a natural explanation for this strange shape and is described here.