VLBI observations of a complete sample of radio galaxies. VI. The two FR I radio galaxies B2 0836 + 29 and 3C 465

We present 5 GHz global VLBI observations of the two Fanaroff-Riley type I radio galaxies B2 0836 + 29 and 3C 465 (2335 + 26). For 3C 465 we present also 1.7 GHz and 8.4 GHz global VLBI data. In addition VLA observations were used to obtain arcsecond-resolution continuum and polarization maps at 5 GHz. Both sources are very asymmetric on the parsec scale, with a core and a one-sided jet, aligned with the main arcsecond-scale jet. We place a limit on the milliarcsecond jet-to-counterjet brightness ratio Bjet/Bcjet ≳ 20 and ≳30 for B2 0836 + 29 and 3C 465, respectively. For 3C 465 the strong asymmetry holds to the kilo-parsec scale. The brightness asymmetry and the ratio between the core radio power and total radio power allow us to constrain the jet velocity close to the core and the orientation of the radio structure with respect to the line of sight. The results suggest that the plasma speed is relativistic on the parsec scale for both sources, i.e., vjet ≳ 0.75c for B2 0836 + 29 and vjet > 0.6c for 3C 465. While vjet decreases from the parsec to the kiloparsec scale in B2 0836 + 29, in 3C 465 the very high vjet holds all the way to the kiloparsec bright spot. Our results are in agreement with the unification scheme suggestion that FR I radio galaxies are the unbeamed population of BL Lac objects. Furthermore, they reinforce the idea that the central engine in FR I and FR II radio galaxies must be qualitatively similar. The different radio morphology could then be due either to an intrinsically different nuclear power, which affects the torus geometry, or to different conditions in the region beyond the parsec scale, where a significant deceleration in the FR I jets occurs.