Published in "Astrophysical Jets", Proceedings of the Astrophysical Jets Meeting, Baltimore 1992, eds. D. Burgarella, M. Livio and C. O'Dea
Abstract. This paper reviews emission-line studies of thermal gas associated with extragalactic jets. Although emission lines contain considerable diagnostic capabilities, such studies have not allowed direct determination of the density and velocity of jets, because the line-emitting gas observed is usually adjacent or external to the jets (in boundary layers or interstellar clouds, for example), rather than being the jet material itself. Emission lines are seen from radiative bow shocks driven by the jets and from interfaces with interstellar gas along the jets. The bow shocks are most clearly seen in the narrow line regions of Seyfert galaxies, where the low jet powers, high ambient densities, and luminous nuclear sources of ionizing photons are all conducive to the formation of dense shells of ionized gas. The potential of future observational and theoretical studies of these bow shocks to improve our understanding of the interaction of jets with dense interstellar gas is discussed. Interface and entrainment effects are commonly seen along the jets of radio galaxies. The emission lines here originate in local gas interacting with the jets and lobes. Jets may be deflected by, accelerate and drive shocks into interstellar clouds. Evidence that the jets entrain gas is discussed. Gas in the outskirts of radio lobes forms an irregular "Faraday screen" and is responsible for depolarization of the radio emission. In general, the emission-line gas in radio galaxies appears to be photoionized, although ionization by shocks or relativistic particles is possible in individual cases. The source of ionizing photons is usually located in the nucleus of the galaxy and probably radiates anisotropically. Two types of anisotropy are recognized, namely wide-angle collimation (arising through either shadowing by a large-scale molecular torus or the intrinsic anisotropy of the emission from a radiation torus) or narrow-angle collimation (resulting from beaming by a relativistic jet). Examples of emission-line gas which may be ionized by such nuclear sources are described. In high redshift radio galaxies and a few nearby ones, the gas appears to be ionized by hot stars, the formation of which was "triggered" by the passage of the jet. Wide-angle, bi-polar gaseous outflows are common in radio-quiet AGN (Seyferts); these outflows may be analogous to the wide-angle outflows seen in molecular gas around pre-main sequence stars. I describe in some detail the kinematics and morphology of the "anomalous arms" of NGC 4258, which represent the most spectacular examples of emission-line jets in a spiral galaxy and illustrate many of the effects suspected in radio galaxies. These jets probably lie close to the plane of the galaxy disk and are interacting strongly with its interstellar medium. The southeast jet in NGC 4258 comprises at least three intertwined, helical strands. This helicity could reflect ballistic outflow from orbiting, supermassive black holes, flow associated with twisted magnetic flux tubes, or fluid dynamical instabilities at the interface between the jet and the interstellar medium. The northwest jet follows a channel between dense, molecular clouds and is clearly deflected by them. Lastly, prospects for future progress in our understanding of jets by means of emission-line studies are discussed.
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