Spectral ageing in the relic radio galaxy B2 0924+30

The radio spectrum of the relic radio galaxy B2 0924+ 30 is investigated using integrated flux densities between 0.151 and 10.6 GHz, as well as images at 0.325, 0.609, 1.400 and 4.750 GHz. Allowing for synchrotron and inverse Compton losses in this source, the observed spectral steepening yields a break frequency of similar to 7 GHz, implying a mean particle age of about 50 Myrs. The corresponding particle advance speed is of the order of 2000 km s(-1). This relatively small particle lifetime following the exhaustion of the central source can explain the paucity of such sources and implies that such objects would easily escape detection if searched for in the cm wavelength regime.