The jets and bow-shocks associated with pre-main sequence stars may be beautifully imaged in a wide range of emission lines (see Reipurth's paper in these proceedings). Little is known about non-thermal radio emission from these stellar jets, but synchrotron emission must be present at some level if only through adiabatic compression of entrained, ambient cosmic rays and magnetic fields. There are no known analogues of such "thermal jets" associated with active galactic nuclei. However, strong narrow line emission (Ltotal narrow line / Lradio 102-104) is found in Seyfert galaxies and much of it is associated with their radio jets and lobes. The high ratio of thermal to non-thermal power in Seyferts results from the low kinetic powers of the collimated ejecta, the high ambient interstellar gas densities, and the high luminosities of the ionizing photon sources in their nuclei. Emission-line studies of the jet-interstellar medium interaction in Seyferts have been limited by the poor resolution of ground-based optical telescopes. Images and spectroscopic studies with HST are now beginning to reveal the thermal component of their "jets" and bow shocks in some detail.
In powerful radio galaxies, the luminosity ratio Ltotal narrow line / Lradio ranges between 0.1 and 3; however, only a fraction of this narrow line emission is associated with the radio jets themselves. Emission-line gas is found in boundary layers around the jets and in interstellar clouds which have been shocked by the passing jet or expanding radio lobes. Emission-line studies thus provide little direct information on the density and velocity of the jet material itself.
The ionization of the gas associated with radio jets is rather a messy topic. Several mechanisms may act, including collisional ionization in shocks, ionization by relativistic particles, photoionization by locally produced synchrotron radiation, and photoionization by the nucleus. In general, photoionization by the nucleus seems to be the dominant process, although exceptions undoubtedly exist. The nuclear ionizing radiation is probably emitted anisotropically, with two forms of anisotropy being recognized. Wide-angle cones of ionizing radiation can result from either shadowing of an intrinsically isotropic source by a dusty torus or an intrinsic anisotropy in the emission process. The latter is expected if the ionizing source is a radiation-pressure supported accretion torus. Narrow-angle anisotropy of ionizing radiation results from relativistic beaming. Emission-line nebulosities ionized by both these kinds of anisotropic ultraviolet continua seem to be present in radio galaxies.
Future ground-based studies of optical emission lines associated with jets should attempt to measure the temperature sensitive lines [OIII]4363 or [NII]5755. Low electron temperatures are the cornerstone of our preference for photo- over collisional ionization. When measured in the extended emission-line regions associated with AGN, the electron temperature is generally found to be in the range 13,000 < Te < 22,000K, much too low for collisional ionization, but higher than the values of Te 11,000K predicted by the best photoionization models (Tadhunter, Robinson & Morganti 1989). Of the various possible explanations for this discrepancy, sub-solar abundances or additional energy input by shock waves are the most promising. Further observations and modeling of this phenomenon should receive high priority. If the temperature is elevated by shock wave heating, an important new probe of the gaseous outflows in AGN would be available. It would be interesting to know if the emission-line gas associated with jets exhibits such elevated temperatures.
The uv and X-ray emission-line spectra of jets are unknown. After correction of the spherical aberration, HST should allow significant studies of jets in the former waveband. Searches for uv synchrotron radiation from jets are an important HST program, not only because of the direct information provided on the acceleration of cosmic ray electrons to 106-107, but also through the ability of such searches to constrain "in situ" photoionization models. Thermal emission from gas in the temperature range 106-108K should be a ubiquitous property of jets given their high velocities and entrainment of surrounding gas. ROSAT may provide the first significant sample of soft X-ray detections of radio jets. Emission-line spectroscopy of such gas must, however, await the sub-arc second spatial and high spectral ( / up to 1000) resolutions of AXAF-I.
I am grateful to Stefi Baum and Chris O'Dea for their comments on the manuscript.