7. OPEN PROBLEMS
Much progress has been made in modeling extragalactic jets in the last
few years owing to (a) the quantitative and qualitative
enrichment of the statistical sample of detailed multifrequency
observations and (b)
the development of reliable numerical codes to simulate the microphysics
of supersonic and relativistic outflows. The basic results definitively
acquired at present are as follows:
- Jet acceleration and collimation take place in the inner
regions ofAGNs through processes in which magnetic fields anchored in
accretion disks are the fundamental elements. The disk-jet connection is
also operating in other astrophysical conditions, such as binary active
stars and star formation regions.
- Jet propagation survives dynamical and kinetic instabilities
due to the interaction with the ambient medium owing to nonlinear effects
that create turbulent boundary layers and overpressured cocoons around
- Jet morphologies can be interpreted in terms of the
above instabilities connecting shock formation, suprathermal particle
acceleration, and synchrotron emission with bright knots, hot spots, bow
shocks, and cocoons.
The distinction between FR I and II radio galaxies is related to the energy
dissipation inside jets, which is parameterized in terms of the flow Mach
number and the density contrast with the ambient medium (or cocoon).
- Doppler beaming is involved in the strong variability
of quasars and blazars and may also explain the one-sidedness of strong jets
and the global energetics of
AGNs. This result is, however, still preliminary.
On the other hand, many questions remain
open, and more investigations are needed to settle the physics governing
- Does the bulk flow contain an ion/electron plasma or
anelectron/positron pair plasma? Originally, the pair plasma was seen as
a factor in reducing global energetic demands. Now the question is how
efficiently relativistic bulk flows can be produced.
- How do accretion disks launch collimated flows? Magnetized
coronae heated by reconnection of loops buoyant from the accretion disk are
perhaps the best candidates, but other processes have not been fully
yet, in particular those involving electromagnetic forces and currents. Nor
has much progress been made on the (formidably difficult indeed) study of
the electrodynamics of black hole magnetospheres.
- Is a fluid approximation a reliable way to represent
the microphysics of jets? Most likely not: Kinetic effects define the
transport coefficients and the development of perturbations and
nonlinear structures inside the flow. Therefore, the boundary layer at
the interface between jets and ambient medium is certainly governed by
these coefficients, and so far we have only preliminary indications
about their effects on mixing, entrainment,
turbulence, etc. A fully kinetic treatment is at present prohibitive; the
next possible step may be a two-fluid model implemented in numerical codes
that have been adapted to the problem.
- How important are currents? Given the high conductivity
of astrophysical plasmas, the general trend is to neglect charge separation
and current effects on large scales. Low return currents can be dispersed
over large cocoons with low current densities without perturbing the ambient
medium. However, current densities may be very high inside jets and may
again influence the transport coefficients and dissipation
processes. The above-mentioned two-fluid approach might answer this
- We have very little information about the spatial and
spectral distributions of the suprathermal component of relativistic
and ions. From the theoretical side, not much effort has been made so far
to couple the suprathermal and thermal components and to couple both of them
to the emitted radiation.
If a conclusion can be drawn at this stage
of modeling astrophysical jets, it might be said that, while 10 years ago
most of what had been observed was interpreted at a phenomenological level,
today we have a more quantitative understanding. The general picture is
rather firm, but most of the microphysics involved are still
This work was supported by grants of
the Italian Ministero dell'Università (MURST) at the Osservatorio
di Torino and by hospitality at the University of California at Irvine and
at the Department of Astronomy & Astrophysics of the University of
The author wishes to thank many colleagues who have shared their interest
in the challenging questions of astrophysical jets, in particular, Gregory
Benford, Gianluigi Bodo, Silvano Massaglia, Robert Rosner, Paola Rossi,
Edoardo Trussoni, and Kanaris, Tsinganos.