PKS B1545−321: bow shocks of a relativistic jet?

Sensitive, high-resolution images of the double-double radio galaxy PKS B 1545-321 reveal detailed structure, which we interpret in light of previous work on the interaction of restarted jets with pre-existing relict cocoons. We have also examined the spectral and polarization properties of the source, the colour distribution in the optical host and the environment of this galaxy in order to understand its physical evolution. We propose that the restarted jets generate narrow bow shocks and that the inner lobes are a mixture of cocoon plasma re-accelerated at the bow shock and new jet material re-accelerated at the termination shock. The dynamics of the restarted jets implies that their hotspots advance at mildly relativistic speeds with external Mach numbers of at least 5. The existence of supersonic hotspot Mach numbers and bright inner lobes is the result of entrainment causing a reduction in the sound speed of the pre-existing cocoon. The interruption to jet activity in PKS B1545-321 has been brief - lasting less than a few per cent of the lifetime ∼(0.3-2) x 10 8 yr of the giant radio source. The host galaxy is located at the boundary of a large-scale filamentary structure, and shows blue patches in colour distribution indicative of a recent merger, which may have triggered the Mpc-scale radio galaxy.