VLBI OBSERVATIONS OF A COMPLETE SAMPLE OF RADIO GALAXIES .6. THE 2 FR-I RADIO GALAXIES B2-0836-465(29 AND 3C)

We present 5 GHz global VLBI observations of the two Fanaroff-Riley ty pe 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 ob servations were used to obtain arcsecond-resolution continuum and pola rization 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 arcseco nd-scale jet. We place a limit on the milliarcsecond jet-to-counterjet brightness ratio B-jet/B-ejet greater than or similar to 20 and great er than or similar to 30 for B2 0836 + 29 and 3C 465, respectively. Fo r 3C 465 the strong asymmetry holds to the kiloparsec scale. The brigh tness asymmetry and the ratio between the core radio power and total r adio power allow us to constrain the jet velocity close to the core an d the orientation of the radio structure with respect to the line of s ight. The results suggest that the plasma speed is relativistic on the parsec scale for both sources, i.e., v(jet) greater than or similar t o 0.75c for B2 0836 + 29 and v(jet) greater than or similar to 0.6c fo r 3C 465. While v(jet) decreases from the parsec to the kiloparsec sca le in B2 0836 + 29, in 3C 465 the very high v(jet) holds all the way t o the kiloparsec bright spot. Our results are in agreement with the un ification scheme suggestion that FR I radio galaxies are the unbeamed population of BL Lac objects. Furthermore, they reinforce the idea tha t the central engine in FR I and FR II radio galaxies must be qualitat ively 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 s cale, where a significant deceleration in the FR I jets occurs.