It is expected that, in most cases, the rate of advance of the head of
the jet is
supersonic, so a bow shock wave will be present. However, the
visibility of the bow
shock is strongly dependent on the physical parameters. Conditions
favorable to the
visibility of the bow shock are: (i) a jet of sufficiently low power
that the bow shock is
slow enough for the postshock gas to cool to
104 K in less
than the dynamical time,
(ii) a high density interstellar or intergalactic medium (which leads to
fast cooling
and a dense, post-shock shell), and (iii) the presence of a luminous
nuclear source of
ionizing photons which will prevent the post-shock gas from cooling
below 104 K
and recombining. All of these conditions are found in Seyfert
galaxies. Extended,
high excitation emission-line gas in Seyferts tends to be aligned, and
co-spatial, with the "linear" radio sources (e.g.,
Haniff, Wilson & Ward
1988).
Further, the emission-line
gas is found to be kinematically disturbed by the radio components
(e.g.,
Whittle et al. 1988).
Unfortunately, these nebulosities are usually too compact
( few arc secs
or few hundred pc scale) for ground-based optical observations to
separate emission
from physically different components (such as the bow shock or a
jet). In a few cases,
however, bow shock-shaped morphologies are seen in both the radio
synchrotron and optical emission-line distributions. Examples include M51
(Crane & van der
Hulst 1992;
Cecil 1988),
as shown in Fig. 1,
NGC 1068
(Wilson & Ulvestad
1987;
Cecil, Bland & Tully
1990),
and NGC 3516
(Miyaji, Wilson &
Pérez-Fournon 1992).
Pedlar et al. (1985)
and Taylor et
al. (1992)
have developed models in which the expanding or
outwardly moving radio components sweep up shells of gas which are
photoionized by the uv continuum from the Seyfert nucleus. In related work,
Wilson & Ulvestad
(1987)
modeled the radio emission of the NE lobe of NGC 1068 as synchrotron
radiation from ambient magnetic fields and cosmic rays which have been
swept up and compressed
by a radiative bow shock driven into the interstellar medium by a
jet. Emission-line gas is also found associated with this bow shock
(Cecil, Bland & Tully
1990).
The essential difference with the model of Pedlar et al. concerns the origin
of the radio
emission: in their model the radio emission comes from a classical radio
lobe or jet,
rather than compressed interstellar material. I conclude this Section
with a summary
of some current questions regarding bow shock models for the kpc-scale
line emission of AGN.
|
Figure 1. (from
Cecil 1988).
Images of the circumnuclear region of M51. The top
grayscale image represents the emission in
[NII] |
Observations (a) What are the precise relative locations of the radio synchrotron and emission-line gases? A combination of images in emission lines from HST and high resolution radio maps can probably answer this question. Such observations should resolve the issue of whether the radio emission comes from close to the shock front itself (implying active particle acceleration at the shock, with the emission-line gas displaced a cooling length downstream), from the post-shock cooling zone, or from a classical radio lobe or jet (with the shocked interstellar gas around it). (b) What kind of physical conditions pertain in the post-shock gas? Emission line ratios should be measured at high spatial resolution (to separate the bow shock related emission from other components) to determine the gaseous density, electron temperature and excitation.
Modelling (c) Is "passive" compression (i.e., without Fermi acceleration of relativistic electrons at the shock) of ambient disk magnetic fields and cosmic rays sufficient to account for the synchrotron powers observed? (d) What are the key emission-line diagnostics of gas which has passed through a radiative shock, but is continuously illuminated by an external, hard, uv continuum (from the nucleus)? Intuitively, one might expect emission-line ratios similar to those of a classical "Seyfert 2" spectrum (for which ionizing radiation is considered to be the sole energy input), but with an elevated electron temperature. (e) Is there evidence for super-solar abundances, as might be expected for ambient gas in the central regions of a spiral galaxy?
Theory (f) Is it feasible to study the evolution of the jet and bow shock with 3d radiative hydrocode calculations, which cover a wide range of Mach numbers and density ratios and also include the important effects of nuclear photoionization? (g) Under what conditions are the dense post-radiative shock shells unstable (cf. Innes et al. 1987; Gaetz et al. 1988; Blondin, Königl & Fryxell 1989)?
6583 at +100 km
s-1 with respect to
systemic. Overlaid on this image are radio contours at