MILLIARCSECOND POLARIZATION STRUCTURE OF 24 OBJECTS FROM THE PEARSON-READHEAD SAMPLE OF BRIGHT EXTRAGALACTIC RADIO-SOURCES .2. DISCUSSION

The observations (Paper I) of the milliarcsecond-scale linear polariza tion properties of compact extragalactic radio sources from the Pearso n-Readhead sample are discussed, In general, the linear polarization p roperties correlate strongly with optical identification, and, for the sources in this sample, galaxies, quasars, and BL Lacertae objects ar e quite distinct None of the compact radio sources identified with gal axies was detected in polarization on milliarcsecond scales. In contra st, most quasars and BL Lacertae objects have polarized structures, es pecially if they have prominent jets. We find systematic differences b etween quasars and BL Lacertae objects in the core polarizations, the orientations of the inferred magnetic fields in the jets, the visible lengths of the jets, and the behavior of the run of fractional polariz ation along the jets. Many of these differences may be caused by diffe rences in the parsec-scale environment in quasars and BL Lacertae obje cts, such as in the density of the line-emitting gas. In BL Lacertae o bjects the inferred magnetic fields in the milliarcsecond scale jets a re perpendicular to the jet axes, while observations of the best resol ved quasars indicate that the fields are often quite closely parallel to the jet axes. This shows that BL Lacertae objects are not simply qu asars whose jets make a particularly small angle to the line of sight. nor are they microlensed images of more distant quasars. There is als o an anticorrelation between the degree of polarization and the total intensity of quasar jet components. Together, these results suggest a picture in which the knots of emission in the jets are associated with transverse shocks acting on a jet containing a mainly tangled magneti c field. In quasars the jets also contain a significant longitudinal c omponent whose relative strength increases with distance from the core . If this is the case, the shocks in quasars must be weak so that the transverse field amplified by the shock never dominates the polarizati on. Our observations suggest that, in general, the magnetic field orie ntations in the milliarcsecond jets continue to scales intermediate be tween those resolved by VLBI and those accessible to the VLA.