Copyright © 1984 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 1984. 22:
319-58 Copyright © 1984 by Annual Reviews. All rights reserved |
5.1. Optical and Infrared Wavelengths
5.1.1 CONTINUUM Optical continuum emission coincides with bright knots in the radio jets of 3C 31 and 3C 66B (52), M87 (249, and references therein), 3C 273 (2, 141, and references therein), 3C 277.3 (159), and possibly Cen A (34). The spectral index between 4500 Å and 5 GHz is generally within 0.1 of 0.7 (52). The M87 knots all have essentially the same connected optical-infrared-radio spectra, with slopes in the radio, infrared, and optical of ~ 0.6, 0.8, and 1.7, respectively; the latter spectral break occurs near 6000 Å in every bright knot (150, 165, 237, 244). In 3C 273 the radio jet brightens toward its tip (64, 181) but the optical jet is more uniformly bright, except for knots at each of its bends (141), so the spectrum above 5 GHz steepens with distance from the QSO (cf. Section 4.2).
The optical continuum is up to 20% linearly polarized in M87 (226, and references therein) and ~ 14% linearly polarized in 3C 277.3 (158, 159). This polarization, the positional coincidence with the radio knots, and the connected optical-radio spectrum in M87 are evidence that the optical continuum is synchrotron radiation from the same regions as the radio jets. The overall linear polarization of the optical jet in 3C 273 is only 3.7 ± 4.1% (226), whereas the radio jet is ~ 12% polarized at both 1.4 and 5 GHz at 2" resolution (Table 1, ref. P2). The radio E vectors in the "head" and "tail" of this jet are nearly perpendicular, however, so high-resolution optical polarimetry is needed to test whether the optical emission is synchrotron radiation there also. If Bj is near equipartition in these jets, the electrons radiating at optical wavelengths are several synchrotron lifetimes from the radio cores. The distribution of optical emission marks the sites of relativistic electron reacceleration, or possibly of pitch angle scattering (239, 240), better than the radio data because of the much longer synchrotron lifetimes of the radio electrons. Studies of optical jets with the Space Telescope will show how discrete, or continuous, the reacceleration regimes are.
5.1.2 EMISSION LINES Data on emission lines from the vicinity of radio jets are reviewed in (158) and (260). The line-emitting regions generally lie beside or beyond the jets, particularly on the outer edges of bends, and are often brightest near, but not at, knots or hot spots. The spectra are not those of normal H II regions, but instead resemble those of Seyfert 2 nuclei. Typical densities are~ 102 to 103 cm-3 and temperatures ~ 20,000 K, leading to pressures near the lower limits to pj in adjacent radio features. Typical bulk velocities are a few hundred kilometers per second, and line widths are 300-500 km s-1. In Cen A, some emission-line filaments beyond the jet have internal differential velocities up to 800 km s-1 (106); they cannot stay intact bug enough to have been convected out from the nucleus. The line strengths suggest photoionization by a power-law continuum, and possibly also shock heating, with different mixtures of excitation mechanisms in different sources. The occurrence of this extranuclear line emission at the edges of radio features, and the increase of line widths toward them (116), suggest interaction between jets and the ambient ISM. The line-emitting gas may be clouds in normal galactic rotation that have encountered jets, becoming heated, ionized, and accelerated by them (26, 73, 74, 116, 158, 159, 282). The uncertain dynamics of the emission-line gas preclude using its bulk velocities to infer jet velocities directly, as suggested in (159).
A continuum and emission-line feature in DA 240 has been reported as an "optical jet" blueshifted relative to the galactic nucleus by 3400 km s-1 (40, 41). This has been invoked (41, 246) as direct evidence that vj = 3400 km s-1 in DA 240, but the feature is not a radio jet and may be a confusing spiral galaxy (261).
Three radio jets are known to be X-ray sources - M87
(230),
Cen A
(228),
and 3C 273
(276). [See
(90)
for a review and further references.] The region near the M87 jet has a
luminosity of ~ 1041
erg s-1 in the Einstein HRI band. Individual knots are not
resolved,
but this integrated X-ray luminosity is consistent with extrapolating
the steep spectrum of the knots above 6000 Å to the X-ray regime. If
the synchrotron interpretation favored in
(230)
is correct, electrons with
107.3 are
required to produce the observed X-rays in the
equipartition magnetic field of the knots; this provides a severe test
for particle acceleration models. The radiative lifetimes of such
electrons would be 
200 yr,
comparable to the light crossing time in
the knots, but much less than the light travel time to the knots from
the nucleus of M87. The X-ray and radio structures of the jet in
Cen A are also very similar
(48),
suggesting that this is also synchrotron
emission, though the case is not as strong as for M87
(90). The
detection of the X-ray jet in 3C 273 depends heavily on deconvolution
of the point-source response from the data, so it has not yet been
analyzed in detail.