Annu. Rev. Astron. Astrophys. 1984. 22: 319-58
Copyright © 1984 by . All rights reserved

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3.1. Sidedness

Most extended extragalactic sources have lobes of similar powers and sizes on each side of the parent object, but not all jets have detectable counterjets. We denote by calS the ratio of intensities between the brighter and fainter jet measured at the same distance from the parent object at low transverse resolution (to minimize the influence on calS of differences in their expansion rates). We classify as one sided those for which calS > 4 wherever the dynamic range of the map allows this to be determined, and as two sided those for which calS < 4 everywhere. We use calS = 4 as the break point solely because it separates two equally numerous groups of jets on available maps, not because it has special physical meaning.

Most jets are one-sided close to their parent object. Jets in weak radio galaxies (e.g. Figure 1) become two sided after a few kiloparsecs; the one-sided bases are typically < 10% of their length, and the jet with the one-sided base is generally somewhat brighter on the large scale. Table 1 (Column 6) classifies the sidedness of the outer 90% of the jets - "1" if calS > 4, "2" if calS < 4, and "B" if calS > 4 at some distances from the core but calS < 4 at others. Most jets in sources with Pcore5 leq 1023 W Hz-1 or Ptot1.4 leq 1024.5 W Hz-1 are mainly two sided, while most jets in more powerful sources, whether radio galaxies or QSRs, are entirely one sided. The value Ptot1.4 approx 1025 W Hz-1 marks the transition between the structural classes I and II of Fanaroff & Riley (86; henceforth FR): the jets in FR I sources (weak, edge darkened, lacking hot spots) are mainly two sided, while those in FR II sources (powerful, edge brightened, strong hot spots) are one sided. All jets in QSRs, whether core dominated or lobe dominated, are entirely one sided.

3.2. Magnetic Configuration

Degrees of linear polarization up to 40% are common in radio jets at centimeter wavelengths (e.g. 19, 99, 186, 191, 220, 248, 263), and local values > 50% are not unusual (183, 184, 256, 278). Jets with low (< 5%) polarization at centimeter wavelengths (31, 47) are exceptional. High degrees of polarization imply ordering of the component of the jet magnetic field Bj perpendicular to the line of sight, but this order need not be three dimensional (137). Field ordering is seen directly on well-resolved maps of the "apparent" (synchrotron emissivity weighted) field Ba derived from multifrequency polarimetry. Three magnetic configurations are common in the 40 jets for which adequate polarimetry exists:

  1. B||, i.e. Ba is predominantly parallel to the jet axis all across the jet.

  2. Bperp, i.e. Ba is predominantly perpendicular to the jet axis all across it.

  3. Bperp-||, i.e. Ba is predominantly perpendicular to the jet axis at the center of the jet, but becomes parallel to the axis near one or both of its edges.

Most two-sided regions of straight jets have either the Bperp or the Bperp-|| field configuration, while most one-sided regions of jets have the B|| configuration (27, 277). In most straight jets in sources with Pcore5 < 1023 W Hz-1 (or Ptot1.4 < 1024.5 W Hz-1), Ba turns from B|| to Bperp or Bperp-|| in the first 10% of the jet, but most jets in more powerful sources are B||-dominated for their entire length (27, and A.H. Bridle, in preparation). This observation, in combination with those of Section 3.1, identifies two main classes of (straight) radio jet - two-sided Bperp- or Bperp-||-dominated jets in weak FR I sources (edge darkened, no hot spots) and one-sided B||-dominated jets in powerful FR 11 sources (edge brightened, stronger hot spots).

Two departures from this trend may arise when jet flows are perturbed:

1. Two-sided jets often have the Bperp-|| configuration where they bend. The B|| edge is often deeper (and more strongly polarized) on the outside of the bend [e.g. 3C 31 (42, 245), NGC 6251 (183), M84 (Table 1, ref. L2)], as if B|| is amplified there by stretching and shearing (4, 208). The bent jets in the C-shaped head-tail source NGC 1265 are B||-dominated even though they are two sided (Table 1, ref. O1). Such fields may be an extreme example of the Bperp-|| configuration resulting from viscous interaction with the ambient medium.

2. Some knots in one-sided jets have Bperp-fields although fainter emission near them is B||-dominated [e.g. knot A in M87 (172), a knot 50" from the core of NGC 6251 (183), and knot A2 in Cen A (48)]. These "magnetic anomalies" at bright knots may be due to oblique shocks that accelerate relativistic particles and amplify the component of Bj parallel to the shock. The fields also become dominated by Bperp components where one-sided B|| jets end at bright hot spots, and the physics there may be similar.

The degree of polarization is generally highest at the edges of B||-regions, but near the centers of Bperp-regions. Two three-dimensional field configurations can fit these polarization distributions: (a) tangled field loops confined to a plane perpendicular to the jet axis near the center of the jet (136, 137) but stretched along the axis toward its edges (21, 183), or (b) "flux ropes" with helical fields of variable pitch (53) and some random component (183).

3.3. Size, Curvature, and Misalignments

Jets in weak galaxies (Figure 1) and in powerful core-dominated sources (Figure 5) are generally short (< 10% of all jets in sources with Pcore5 < 1022.5 W Hz-1, and only 13% of those in sources with Pcore5 > 1026.5 W Hz-1, have dj > 40 kpc, but ~ 50% of those in sources of intermediate powers exceed this length). The jets in core-dominated sources may be shortened by projection effects if the cores are Doppler boosted (Section 6.1.7), but those in weak galaxies are two sided (e.g. Figure 1) and so are probably short intrinsically. Strong jet curvature is also common at the two extremes: C-shaped jets in weak "head-tail" cluster galaxies, and jets in powerful core-dominated sources (Figure 5). The curvature in head-tail sources is probably due to bending by ram pressure (10, 128); that in powerful sources may be due to confinement or to wandering of the central collimator (see Section 6.2.2).

The misalignments between parsec- and kiloparsec-scale jets, summarized in (200), increase with core prominence fC. Several lobe-dominated radio galaxies with kiloparsec-scale jets have cores with one-sided parsec-scale jetlike extensions on the same side as the large jets (Section 6.2.3) and aligning with them to leq 10° (56, 67, 132, 142, 197 - 199, 209; Figure 2). In core-dominated sources, however, parsec- and kiloparsec-scale structures are often misaligned by > 20° (57, 69; Figure 4), and in 3C 345 and 3C 418 (Figure 5) by > 90°. The jets in core-dominated sources bend continuously, with the sharpest curvature occurring closest to the core (58 - 60, 200, 274; Figure 5). These data are consistent with the short jets in core-dominated sources being close to the line of sight - small bends in jets near the line of sight may project as large apparent misalignments (e.g. 201).

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