The most basic parameters describing a jet are its mass per unit length
(m), mass
flux (J), momentum flux (or thrust, T) and energy flux (or
power, K). These
parameters may be expressed in terms of the density
(
), velocity
(v), internal energy,
total stress tensor, heat flux density, gravitational potential and area
(S) of a surface perpendicular to the jet (e.g.,
Leahy 1991).
In most jet models,
the momentum and
energy fluxes are dominated by the kinetic motion of the jet material,
in which case
m = 
dS,
J =
vdS,
T =
v2
dS, and K =
v3
/ 2dS. Thus the problem of
finding these parameters comes down to determining
and v. It
is unlikely that
theorists will be able to construct realistic models of jets until
and v have been directly
measured by observers, ideally as close as possible to the
central engine and also as a
function of distance along the jet (to study, for example, entrainment
effects).
Extragalactic jets are observed primarily as radio synchrotron sources
but such observations
provide little direct information on the density and velocity. A number
of indirect
arguments, however, favor light, fast jets. (i) Because of the steep
dependence of the power
on v, luminous radio sources are most easily accommodated with
fast, probably
relativistic, jet flows; the mass loss rate and total mass loss from the
nucleus over the radio
source's lifetime are then minimized and there is no conflict with the
(mostly) upper
limits on from
Faraday depolarization arguments. (ii) The existence
of "hot spots"
and "cocoons" of old jet material implies that the rate of advance of
the head of the jet
is substantially less than v, so
<
e, where
e is the
external density
(Scheuer 1974).
(iii) In a qualitative way, it seems that nonthermal processes are
likely to be enhanced
over thermal ones when speeds are high and densities low. (iv) If the
jet is surrounded by a "cocoon" of backflowing jet material
(Norman et al. 1983),
surface instabilities
should be less destructive to the jet than if it were surrounded by
interstellar or
intergalactic gas, for the "cocoon" is of low density like the jet. (v)
Attempts to estimate
v by dividing the (model dependent) jet power by the (model
dependent) jet thrust
tend to give high speeds for powerful sources like Cygnus A
(e.g.,
Leahy 1991).
(vi) Models of "bent" beams in narrow angle tail sources typically provide
upper limits to the particle density in the jet of
10-6 - 10-4
cm-3
(O'Dea 1985).
On the other hand,
the morphologies of "bent" jets have also been successfully modeled as
ballistic motion
disturbed by nuclear precession or acceleration of the active galaxy by
the gravity of
a companion galaxy; such models (e.g.,
Blandford & Icke 1978)
give high gas densities
and relatively low jet speeds, of order hundreds to thousands km
s-1. In SS433, the
jet velocity of 0.26c can be measured directly from the hydrogen lines,
which originate
in dense gas. Lastly, the difference between Fanaroff-Riley (FR) classes
I and II radio morphologies has challenged the theorist (e.g.,
Norman et al. 1984;
Bicknell 1986)
and is often accounted for in terms of higher velocities in the FRIIs
(e.g.,
Williams 1991);
recent progress in this area is reviewed by Laing in these proceedings.
About a decade ago, some astronomers hoped that studies of optical
emission lines would provide definitive values for
and
v. Unfortunately, this expectation has not
been met. In general, the jet material itself appears to be of too low
density and/or
too hot to emit significant optical line emission, and the emission
lines that are seen
originate in interstellar or intergalactic gas close to the
jet. Therefore, emission-line
studies provide no direct information about the jet
material. Nevertheless, observations
clearly indicate that interstellar and intergalactic gas may be
compressed, ionized or
excited by a jet. I shall separate the emission-line observations into
two groups, namely
those that may be interpreted in terms of radiative bow shocks driven by
the head of
the jet into the surrounding gas (Section 2)
and those that are suggestive of interface
and entrainment effects along the jet
(Section 3). In practice, the
distinction between
these two categories is not always clear cut (e.g., jets may
drive shocks into interstellar
clouds encountered along their path, not just at the current jet
terminus). I shall then
go on to discuss in more depth the ionizing source for this gas
(Section 4). In
Section 5, evidence for wide angle
gaseous outflows in active galaxies will be described; perhaps
these outflows are analogous to the wide-angle outflows sometimes seen
from pre-main
sequence stars. Section 6 summarizes a
recent emission-line study of the intertwined,
helical jets in the spiral galaxy NGC 4258. Concluding remarks and a
brief discussion
of future prospects for emission-line studies of extragalactic jets are
given in Section 7.