Exoscales-sheaths around FR I and FR II jets

next up previous
Next: Endoscales-More Signs of Up: Tranverse Scales Previous: Tranverse Scales

8.1 Exoscales-sheaths around FR I and FR II jets 

Distinct emission excesses around, and approximately coaxial with, bright radio jets were mentioned at Green Bank (Bridle 1984; O'Dea & Owen 1984). New evidence for this phenomenon was shown here for several sources. (After much debate, a group of the observers at this meeting agreed to call such features  ``sheaths''-unwittingly reinventing the term used at Green Bank by O'Dea & Owen!)

Rudnick & Katz-Stone (these Proceedings) used ``spectral tomography''  to show that the jets in the FR I radio galaxy 1231+674  have two spectrally-distinct components with different transverse scales. Varying proportions of ``inner jet'' and ``sheath''  account for spectral gradients that would otherwise be attributed to aging. As well as providing new evidence for FR I jet sheaths,  their result emphasizes the importance of exploring multi-frequency visualizations of both intensity and spectral index before choosing how to model the spectral properties of radio sources.

A miscellany of aligned ridges, filaments,  rings and some diffuse emission was noted within a few radii of two FR II jets-Cygnus A  (Katz-Stone & Rudnick 1994; Rudnick & Katz-Stone, these Proceedings) and 3C353  (Swain, Bridle, & Baum, these Proceedings). The reality of these features is clear, but searches for intensity, spectral,  or polarization  relationships between such putative ``sheath''  features and the jets (see Swain, Bridle, and Baum) are needed to test whether they are truly jet-related. Such relationships might also distinguish whether sheaths  visualize an outer scale of the FR II jet outflow, or a distinct inner scale of the lobe/cocoon/backflow.  The latter possibility might alleviate the ``counterjet candidate" problem in the 3CR quasars, if some of the detections are backflowing sheaths, not outflowing counterjets. 

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