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Title:
Sub-Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. III. Kinematics of Parsec-scale Radio Jets
Authors:
Kellermann, K. I.; Lister, M. L.; Homan, D. C.; Vermeulen, R. C.; Cohen, M. H.; Ros, E.; Kadler, M.; Zensus, J. A.; Kovalev, Y. Y.
Affiliation:
AA(National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 ), AB(National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 ; Karl Jansky Fellow.; Current address: Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907.), AC(National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 ), AD(Netherlands Foundation Research in Astronomy (ASTRON), Postbus 2, NL-7990 AA Dwingeloo, Netherlands; ), AE(Department of Astronomy, MS 105-24, California Institute of Technology, Pasadena, CA 91125; ), AF(Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany; ), AG(Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany; ), AH(Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany; and National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 ), AI(National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944; and Astro Space Center of P. N. Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia; )
Publication:
The Astrophysical Journal, Volume 609, Issue 2, pp. 539-563. (ApJ Homepage)
Publication Date:
07/2004
Origin:
UCP
ApJ Keywords:
Galaxies: Active, Galaxies: Jets, Galaxies: Quasars: General, Radio Continuum: Galaxies
DOI:
10.1086/421289
Bibliographic Code:
2004ApJ...609..539K

Abstract

We report the results of a 15 GHz (2 cm) multiepoch Very Long Baseline Array (VLBA) program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity that can represent an underlying continuous jet flow, but we also see some stationary and even apparently inward-moving features that coexist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure.

The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the timescale of flux density variations is consistent with a steep power-law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows, and contrary to the assumption of simple unified models, not all sources have intrinsic speeds close to c.

Usually, the observed jet flow is in the general direction of an established jet. However, many jets show significant bends and twists, where the observed motions are nonradial but are aligned with the local jet direction, suggesting that the jet flow occurs along preexisting bent channels. In a few cases we have observed a clear change in the direction of a feature as it flows along the jet. Radio jets that are also strong gamma-ray sources detected by EGRET appear to have significantly faster speeds than the non-EGRET sources, consistent with the idea that gamma-ray sources have larger Doppler factors than non-gamma-ray sources. Sources at high redshift have systematically lower angular speeds than low-redshift jets, consistent with standard cosmologies.

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