The jets and lobes of nearby, extended radio galaxies are ideal laboratories for exploring the spectral and dynamical evolution of the radio emitting plasma that emanates from active galactic nuclei and propagates through the ambient medium. Here, we present a deep, high-resolution radio continuum study of the low-redshift (z=0. 01713), Fanaroff-Riley class I (FRI) radio galaxy 3C 449 performed with a combination of radio data at 145 MHz acquired with the LOw Frequency ARray (LOFAR) and archival Very Large Array (VLA) data at 1365, 1485, 4985, and 8485 MHz. Our LOFAR maps of the source have angular resolution 20 20 (7. 2 kpc times 7. 2 kpc) and 6. 0 6. 0 (2. 2 kpc times 2. 2 kpc), revealing the full extent of the known radio emission (≈ 22 i. e. ≈ 480 kpc) at the highest angular resolution achieved to date. Our spectral index maps show, for the first time, the high-resolution distribution of the spectrum in the 145-8485 MHz frequency range over a source region that extends beyond 2. 5 (54 kpc). The average 145-8485 MHz source spectrum is consistent with a single power law and stays approximately constant over the inner ≈ 50 (≈ 18 kpc) of both source jets. Beyond ≈ 50 both on the northern and on the southern source sides, the higher-frequency spectrum steepens and the spectral break frequency lowers with increasing distance from the radio core, indicating the absence of relevant sites of particle acceleration beyond those distances. In our 145-1365 MHz spectral index map, we detected a flatter spectrum spine surrounded by a steeper spectrum sheath in the inner ≈ 2. 5 (54 kpc) and ≈ 3 (65 kpc) of the southern and northern jet, respectively. Beyond ≈ 1 ≃ 150 Myr. If the latter age were representative of the dynamical source age, the average expansion speed of both jets during the source lifetime would be supersonic, with M≃ 4. 1 and M≃ 2. 8 for the northern and southern jets, respectively. Because numerical magneto-hydrodynamical simulations of FRI jets suggest that the source's current expansion is subsonic, the high average Mach numbers might arise either from the source being highly supersonic for a small fraction of its lifetime or from a severe underestimation of the spectral age. This is either due to particle acceleration on scales of hundreds of kiloparsecs (not detected in our observations) or to the presence of a non-homogeneous magnetic field with a complex structure. the spine-sheath structure was also detected in the 1365-8485 MHz frequency range, confirming previous findings. The steep spectrum sheath may be the signature of interaction between the jet and the ambient medium. By modelling the spectral index maps under the assumption of equipartition and of a constant magnetic field across the source, we derived maps of the highest radiative age of the particles all over the source in a standard ageing scenario. From the oldest radiating plasma, located in the southern radio lobe, we estimated the source spectral age as τ≃ 200 Myr. At the outer edges of both the northern tail and southern lobe, we estimated a spectral age of τ_ ̊m sp
Ricci et al. (Fri,) studied this question.