Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion

Details Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion Milliarcsecond Polarization Structure of 24 Objects from the Pearson-Readhead Sample of Bright Extragalactic Radio Sources. II. Discussion

Oct 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993apj...416..519c&link_type=abstract

Astrophysical Journal v.416, p.519

Astronomy and Astrophysics

Astronomy

92

Galaxies: Bl Lacertae Objects: General, Galaxies: Jets, Galaxies: Magnetic Fields, Galaxies: Quasars: General, Polarization

Scientific paper

The observations (Paper I) of the milliarcsecond-scale linear polarization properties of compact extragalactic radio sources from the Pearson-Readhead sample are discussed. In general, the linear polarization properties correlate strongly with optical identification, and, for the sources in this sample, galaxies, quasars, and BL Lacertae objects are quite distinct. None of the compact radio sources identified with galaxies was detected in polarization on milliarcsecond scales. In contrast, most quasars and BL Lacertae objects have polarized structures, especially if they have prominent jets. We find systematic differences between 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 polarization along the jets. Many of these differences may be caused by differences in the parsec-scale environment in quasars and BL Lacertae objects, such as in the density of the line-emitting gas.
In BL Lacertae objects the inferred magnetic fields in the milliarcsecond scale jets are perpendicular to the jet axes, while observations of the best resolved quasars indicate that the fields are often quite closely parallel to the jet axes. This shows that BL Lacertae objects are not simply quasars whose jets make a particularly small angle to the line of sight, nor are they microlensed images of more distant quasars. There is also 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 magnetic field. In quasars the jets also contain a significant longitudinal component 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 polarization. Our observations suggest that, in general, the magnetic field orientations in the milliarcsecond jets continue to scales intermediate between those resolved by VLBI and those accessible to the VLA.

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