Radio galaxies with a 'double-double' morphology - III. The case of B 1834+620

We present a study of the peculiar radio galaxy B 1834+620. It is characterized by the presence of a 420-kpc large edge-brightened radio source which is situated within, and well aligned with, a larger (1.66 Mpc) radio source. Both sources apparently originate in the same host galaxy, which has an R-s magnitude of 19.7 and a redshift of 0.5194, as determined from the strong emission lines in the spectrum. We have determined the rotation measures towards this source, as well as the radio spectral energy distribution of its components. The radio spectrum of the large outer source is steeper than that of the smaller inner source. The radio core has a spectrum that peaks at a frequency of a few GHz. The rotation measures towards the four main components are quite similar, within similar to 2 rad m(-2) of 58 rad m(-2). They are probably largely Galactic in origin. We use the presence of a bright hotspot in the northern outer lobe to constrain the advance velocity of the inner radio lobes to the range between 0.19c and 0.29c, depending on the orientation of the source. This corresponds to an age of this structure in the range between 2.6 and 5.8 Myr. We estimate a density of the ambient medium of the inner lobes of less than or similar to 1.6 x 10(-30) g cm(-3) (particle density less than or similar to 8 x 10(-7) cm(-3)). A low ambient density is further supported by the discrepancy between the large optical emission-line luminosity of the host galaxy and the relatively low radio power of the inner lobes.