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A. A. West (NRAO \& Haverford College), K. I. Kellermann (NRAO), R. C. Vermeulen (NFRA), J. A. Zensus (MPIfR), M. H. Cohen, (Caltech)
The NRAO Very Long Baseline Array (VLBA) has been used over a period of 4 years to obtain repeated images of a large sample of quasars and active galactic nuclei (AGN) at 15 GHz. The resolution is better than 1 milliarcsecond (mas) and we are able to located the relative position of small features with an accuracy better than 0.01 mas. This corresponds to about 0.1 parsecs at cosmological distances (H = 65 km/sec/Mpc). Thus we are able to determine component motions even for sources with apparent subluminal velocities.
In most quasars and AGN, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c. The median observed apparent velocity is about 5c, corresponding to a typical intrinsic velocity of about 98 percent of the speed of light oriented about 10 degrees to the line of sight. There is some evidence for accelerations along the jet, but no evidence of infall into the central engine As indicated by previous studies, (Vermeulen & Cohen 1994, ApJ, 430, 467; Vermeulen 1995, PNAS, 92, 11385) consideration of the effects of Doppler boosting suggests that the distribution of apparent velocities is not consistent with simple ballistic models or that there is a significant spread in intrinsic velocity.
We have examined the apparent angular velocity-redshift relation which offers a potentially powerful test of world models. Our data are consistent with standard Friedmann models and somewhat favor a deceleration constant between 0 and 1/2 for a zero cosmological constant. Further observations of a bigger source sample, especially at large redshift, along with an increased time base, may lead to more precise constraints on world models.
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