VLBA observations of a sample of nearby FR I radio galaxies

We observed 17 nearby low-luminosity FR I radio galaxies using the NRAO Ver y Long Baseline Array (VLBA) at 1.67 GHz, as part of a multiwavelength stud y of a complete sample of 21 sources Selected by radio flux density from th e Uppsala General Catalogue of Galaxies. We detected radio emission from al l 17 galaxies. At a FWHM resolution of approximate to 10 x 4 mas, five gala xies show only an unresolved radio core, 10 galaxies show core-jet structur es, and two galaxies show twin-jet structures. Comparing these VLBA images with images previously obtained with the NRAO VLA, we find that all detecte d VLBA jets are well aligned on parsec scales with the VLA jets on kilopars ec scales and that the jet-to-counterjet surface brightness ratios, or the sidedness, decreases systematically with increasing distance along the jet. We attribute the sidedness to the Doppler boosting effect and its decline to the deceleration of the jets. We show that a distribution of Lorentz fac tor centered near Gamma = 5 can reproduce our VLBA detection statistics for core, core-jet, and twin-jet sources. We also note that the luminosity per unit length, L-j, of the VLBA jets drops quickly with distance, r, along t he jet, approximately as L-j proportional to r(-2.0). We discuss three diff erent mechanisms to explain this jet fading: (1) the decrease of Doppler bo osting due to jet deceleration, (2) synchrotron losses, and (3) expansion l osses in constant velocity but adiabatically spreading jets. Mechanisms (1) and (2) are inconsistent with the observations, while mechanism (3) is con sistent with the observations provided the magnetic field lines in the jets are aligned perpendicular to the jet axis. This implies that the decelerat ion of the jets required by the unified scheme does not occur on the tens o f parsec scales but must occur on larger scales.