Transport Velocities

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4. Transport Velocities 

There was little doubt at Green Bank that bulk relativistic motions occur in the jets of quasars and of some radio galaxies on parsec scales. There was much debate over whether such motions survive to kiloparsec scales in radio-loud sources of any luminosity, or occur on any scale in most FR I sources. 

In 1984, the case for large-scale relativistic flows came from the prevalent one-sidedness  of jets in quasars and (where detected) in FR II radio galaxies. This same one-sidedness  was also argued as a symptom of intrinsic asymmetries,  however, and ``flip-flop'' models  were debated. The fact that one-sided  jets are more prominent  in quasars than in FR II radio galaxies was adduced (e.g., Bridle & Perley 1984) as evidence that the two might be similar objects seen from different angles. But this idea had to be married to that of an obscuring torus    (Barthel 1989) to launch a ``unified scheme''  for the radio and optical properties of FR II sources. Many lines of evidence now make this scheme attractive, at least in its broad outlines:

  1. the depolarization  asymmetries of FR II radio sources with prominent one-sided radio jets correlate well with the jet sidedness  (Laing 1988; Garrington et al. 1988, 1991),

    FR II quasars,  correlations between the sidedness  (intensity asymmetry) and prominence  (fractional flux density relative to the lobes) of parsec-scale and kiloparsec-scale jets improved as more objects were imaged in detail on both scales (Bridle et al. 1994a; Hough 1994; Wardle & Aaron, these Proceedings).

  2. some narrow-line AGN  have broader emission lines in scattered light, consistent with orientation-dependent obscuration    (e.g., Antonucci 1984; Antonucci & Miller 1985; Antonucci, Hurt, & Kinney 1994).

Orientation-based unification  has also been extended (e.g., Urry & Padovani 1995) to blazars  and FR I sources, for which we have learned that:
  1. they, too, have depolarization  asymmetries, correlated with the sidedness  of the bases of their large-scale jets (Parma, de Ruiter, & Fanti 1996),

  2. their parsec-scale jets generally resemble those of FR II sources, and correlate well in sidedness  with their kiloparsec-scale jet bases (Giovannini et al. 1995; Venturi et al. 1994, 1995; Tingay et al., these Proceedings).

  3. the emission on their outer edges is initially much more symmetric in intensity across the nucleus than that on their axes, where the flow   is expected to be faster (Laing 1993, 1994 and these Proceedings).

So it is not surprising that the debate about the range of scales over which jets have relativistic bulk motions, which energized the Green Bank meeting, was absent here.

Proponents of intrinsic asymmetries in FR II sources were probably not entirely wrong, however---FR II radio galaxies without strong radio jets, which are expected to lie near the plane of the sky and so to show few relativistic asymmetries, have asymmetries in depolarization and in spectral index that correlate with the lengths of their lobes and with optical emission line asymmetries (Liu & Pooley 1991a,b; Pedelty et al. 1989). The relativistic effects may therefore be superposed on intrinsic (environmentally-produced?) asymmetries that correlate with lobe length (e.g., Bridle et al. 1994b).

Discussion of transport velocities  has now shifted towards unifying the pictures of FR I and FR II sources  by asking how relativistic jets decelerate  in galactic environments  (e.g., Bicknell, these Proceedings).

next up previous
Next: ``Speed bumps'' Up: Observations of Energy Transport Previous: The Nearest Engines?

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