Sub‐Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. III. Kinematics of Parsec‐scale Radio Jets

We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in1994 to study the outflow in radio jets ejected from quasars and activegalaxies. The observed flow of 208 distinct features measured in 110 quasars,active galaxies, and BL Lac objects shows highly collimated relativistic motionwith apparent transverse velocities typically between zero and about 15c, witha tail extending up to about 34c. Within individual jets, different featuresappear to move with a similar characteristic velocity which may represent anunderlying continuous jet flow, but we also see some stationary and evenapparently inward moving features which co-exist with the main features.Comparison of our observations with published data at other wavelengthssuggests that there is a systematic decrease in apparent velocity withincreasing wavelength, probably because the observations at differentwavelengths sample different parts of the jet structure. The observeddistribution of linear velocities is not consistent with any simple ballisticmodel. Either there is a rather broad range of Lorentz factors, a significantdifference between the velocity of the bulk relativistic flow and the patternspeed of underlying shocks, or a combination of these options. Assuming aballistic flow, comparison of observed apparent velocities and Doppler factorscomputed from the time scale of flux density variations is consistent with asteep power law distribution of intrinsic Lorentz factors, an isotropicdistribution of orientations of the parent population, and intrinsic brightnesstemperatures about an order of magnitude below the canonical inverse Comptonlimit. It appears that the parent population of radio jets is not dominated byhighly relativistic flows (abridged).Comment: 25 pages, 11 figures, 3 tables, accepted for publication in the Astrophysical Journa