1995A&A...303..427K

We have investigated the distributions of the spectral index across ten low- and intermediate-luminosity radio galaxies selected from the B2 catalogue. Some important source properties pertaining to the propagation and ageing of the relativistic particles have been derived. Most sources exhibit a spectral steepening towards the outer lobe regions, with spectral changes which are basically consistent with synchrotron ageing. However, the steepening is less pronounced as compared to that found for FRII sources. Typical source ages range from about 40 to 70Myr. A few cases deviating from this behaviour are discussed. A peculiar spectral index distribution is found in the giant radio galaxy 0136+39. The very steep spectrum throughout the source is reminiscent of a relict source, although it hosts a rather strong, but steep-spectrum, central source. The spectrum steepens strongly from the outer lobes towards the core, similar to what is seen in FRII radio galaxies. The source 0828+32 possesses a young and an old lobe system. The peculiar morphology of 0828+32 is interpreted in terms of a precessing jet with decreasing length, consistent with the observed behaviour of the radio spectrum along its lobe system. This model is supported by the low upper limits of its core radio luminosity, and suggests that this source is in the process of exhaustion.