Acceleration and collimation near the nucleus

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5.1 Acceleration and collimation near the nucleus     

Velocity differences among features in the same source, and with time in the same feature, are common in parsec-scale jets, (e.g., Pearson, these Proceedings and references therein). 3C345 remains the prime example of a rich parsec-scale velocity field  (Lobanov & Zensus, these Proceedings): both the organization of the trajectories and the pattern speeds  increase away from the nucleus. Increasing jet collimation away from the nucleus is also a key part of Conway & Murphy's (1993) explanation for the ``90°-misaligned'' population (Pearson, these Proceedings; Appl, these Proceedings).

For M87 (Biretta, these Proceedings), a difficult VLA  experiment implying large-scale pattern speeds  ranging from stationary  to superluminal  has been bolstered by HST  data (with completely different systematic errors) also suggesting superluminal motion. This jet also shows increasing pattern speeds between parsec and few-hundred-parsec scales, and improving (angular) collimation with increasing distance from the nucleus.

The velocity fields within parsec-scale jets and the velocity dispersion among different source types are both key factors in testing unified schemes via the statistics of apparent Lorentz factors. Both can now be explored more fully (e.g., Pearson, these Proceedings; Vermeulen, these Proceedings; Daly, Guerra, & Güijosa, these Proceedings) by sensitive imaging of samples that include weaker radio nuclei, and thus should be freer from orientation bias  than earlier VLBI work.

Alan Bridle
Wed Apr 10 10:19:46 EDT 1996