ACTIVE GALAXIES AND QUASISTELLAR OBJECTS, JETS

Jean A. Eilek

Fundamental considerations of the energy supply in large-scale extragalactic radio sources(``radio galaxies'') led astronomers to predict that these sources should contain jets or beams that connect the nucleus of the parent galaxy to the large-scale radio lobes. These predictions were made shortly after the discovery of the sources; their proof had to wait for the advent of radio interferometers. Interferometers produced much higher quality radio images than those available from early radio telescopes; the jets were indeed found to be a major feature of all radio galaxies.

RADIO GALAXIES

Strong, double radio sources were first identified with external galaxies in the 1950s, and in fact the search for optical identifications of radio sources led to the discovery of quasars in the 1960s. Over the following couple of decades, data were accumulated on large numbers of radio galaxies, and a basic picture of these sources emerged. Radio galaxies generally consist of two fairly symmetric radio-luminous ``lobes''; the parent galaxy lies midway between the lobes. The parent galaxy is almost always a giant elliptical, at least at nearby redshifts; it can also be the dominant central galaxy in a cluster of galaxies. At high redshift, double radio sources are found to be associated with quasars (for which the classification of the parent galaxy is still a matter of discussion) and with distorted distant galaxies. Not all active galaxies produce large and powerful radio jets, however. Active nuclei in spirals are much weaker radio sources, which may involve small jets or may only lead to uncollimated core sources. Most QSOs are not strong radio sources; only about 1 in 10 produces the dramatic radio jets which are the topic of this entry.

The radio structure is typically a few to 10 times the sizes of the parent galaxy (ie. , 50-200 kiloparsecs (kpc)]; however, smaller (down to 10 kpc) and much larger (up to 1 Mpc) examples are known. The spectrum and polarization of the radio emission suggest that the radiation mechanism is incoherent synchrotron emission, produced by relativistic electrons spiraling in magnetic fields. Knowledge of the emission mechanisms allows us to estimate the minimum energy contained in the radio lobes (that part of it in relativistic electrons and the magnetic field). This minimum energy, divided by the luminosity, predicts a very short lifetime. Thus, unless we are seeing these sources at a very special epoch, there must be either a great deal of energy hidden in the lobes, or a continuous resupply of energy to the lobes. One way of doing this is through a ``pipeline'' from the galactic nucleus; there jets were therefore predicted fairly soon after radio galaxies were discovered.

Although optical jets had been known in M87 (a nearby ical elliptical galaxy) and in 3C 273 (a quasar) for some time, our ding understanding of jets really grew only after interferometer observations found that jets are common in radio galaxies and in radio-strong quasars. There are now enough data (several hundred radio jets are currently known) to justify studying the basic physics of the jets and to test models against observations.

OBSERVATIONS OF RADIO JETS

This section summarizes important trends in the data. Two examples are shown, Fig. 1 (3C 175, a 200-kpc-long source, associated with a quasar) and Fig. 2 (M87, a 3-kpc-long jet in the core of a nearby galaxy). Nearly all of our information on jets comes from radio observations; only a few jets have been detected at other frequencies (optical and x-ray).

Figure 1. Very Large Array (VLA) image of the radio emission from the quasar 3C 175. The radio source is approximately 200 Mpc end-to-end. The bright central coincides with the optical qausar, which is at a redshift of 0.77. Several knots of enhanced emission can be seen in the jet. The ``snowy'' background in the image is the noise level of the observation. (Courtesy of A. Bridle.)

Figure 2. VLA image of the radio emission from the inner part of the galaxy M87. The jet shown here is about 3 kpc long; it emanates from a source in the core of the galaxy, seen at the left of this image. Thsi jet sits in the center of a nearby elliptical galaxy, which is in the Virgo cluster at a distance of approximately 20 megaparsecs. Complex substructure can be seen in the jet, as well as a sudden transition from well-collimated to more disordered flow. Other radio data (not present here) show that the jet sits inside a large envelope of diffuse radio emission. The ``rings'' around the core and the ``shadows'' above and below the jet are artifacts of the telescope response and the data reduction process. (Courtesy of F. N. Owen.)

Most jets are barely resolved or unresolved, so little can be said in general about substructure within the jets. The few jets that have been resolved, however, show dramatic substructure: bright knots, what appear to be twisted helices of emission, or surface emission (e.g., the jet in M87, Fig. 2). It seems likely that complex substructure will prove to be the rule rather than the exception.

The basic working model of jets is that they represent an outflow of matter and energy from the nucleus. However, there has been no direct measurement of an outflow of jet material (because no emission lines are seen, Doppler shifts cannot be measured). Proper motion of bright knots has been detected. Many parsec-scale nuclear jets have been observed by very long baseline interferometry and motion of bright knots is commonly detected. The knots move away from the nucleus, at apparent velocities (projected on the sky) of 2 to 10 times the speed of light, c. This is believed to result from a speed close to c in a jet lying close to the observer's line of sight; light travel effects can account for the apparent superlight speeds. Proper motion has been searched for from the kiloparsec-scale jet in M87. The data up to 1990 rule out velocities greater than c (again projected on the sky); detection of sublight speed proper motion has not yet been confirmed in this source. It is important to note that these are not necessarily detections of a material flow speed; they could equally well be measuring a pattern speed (such as traveling wave).

Another indirect measure of jet speed is possible: the measurement of side-to-side brightness ratios for the two jets in a particular source. High-power jets are often asymmetric, with sidedness ratios ranging from a few to several tens. This is especially true in quasar jets, but is also seen in high-power radio galaxies.

One possible explanation of this (although not the only one) is that the sidedness is due to relativistic beaming of the emitted radiation. A pair of jets, each moving at a velocity v = c, of equal intrinsic luminosity, will have an apparent brightness ratio given by (1 - cos )^x, where x lies between 2 and 3 (depending on the spectrum of the radio emission) and is the angle between the jet and the observer's line of sight. Thus, for cos = 1 (i.e.: for jets moving with v c and lying close to the line of sight), strong side-to-side asymmetrics are seen.

This can, in principle, be used to estimate the average jet velocity in a set of radio galaxies or quasars. If one has a set of jets whose orientation (relative to the line of sight) is known to be random (the ``parent sample''), their average sidedness will provide a measure of their average velocity. This type of analysis has been attempted, using different sets of observations, and the mean lorentz factor of the jets [ = (1 - ²)^-1/2] has been estimated at between 2 and 10. This analysis is not yet conclusive, however; several problems remain. First, it is not yet clear that a proper parent sample exists, so that the statistical arguments may not be conclusive. Second, it has not been established that the detailed jet structures seen in high-quality images are consistent with highly relativistic bulk flow. Third, the deprojected length of some of the one-sided quasar jets must be several megaparsecs, which is larger even than clusters of galaxies; this may be an awkward consequence of the model. Thus the question of sidedness is tantalizing, but has not yet produced conclusive evidence on the jet speed.

THE PHYSICS OF RADIO JETS

Work on physical models of the jets is hampered by the fact that none of the parameters that are likely to be important can be measured directly. We have little information on the density, internal energy, composition, velocity, or magnetic field in the jet material. (Note that the synchrotron emissivity, which we can measure, is a nonlinear function of the density and energy of the relativistic electrons, and of the magnetic field. Early work hoped that detection of Faraday rotation would provide independent information on the gas density and magnetic field; however, almost all Faraday rotation detected so far turns out to come from thermal gas surrounding the radio source. ) We can, however, list some general considerations that any model of the jets must satisfy.

The total energy supplied to the lobes, over the lifetime of the source, must be at least as large as the current minimum energy contained in the lobes plus the net energy radiated over the lifetime of the source (perhaps 10⁶⁰ erg; this estimate depends on the details of the model as well as on the estimated lifetime). The momentum flux transported by the jet must be large enough to push the extragalactic gas aside and establish the current size of the radio source. The direction of the flow must remain nearly constant over the age of the source (at least 10⁷ yr, perhaps much longer).

The internal state of the plasma in the radio jet and lobes must be consistent with the observed synchrotron luminosity. The relativistic electrons are probably transported out from the galactic nucleus, and may also be reaccelerated locally in the jet or lobes. These sources are, in fact, distant laboratories for studying charged-particle acceleration. The same mechanisms that accelerate cosmic rays, and the energetic particles in solar flares and supernova remnants, are likely to be operating in radio galaxies. Shocks and turbulence in the radio source plasma are currently thought to be the most likely mechanisms for accelerating the particles.

Models of the sources must also maintain the magnetic field strength necessary to produce the observed radio synchrotron emission. Because the plasma is highly conducting, induction will reduce magnetic fields in the nuclear plasma to very low levels as this plasma expands into the jet and lobes. It is very likely, therefore, that turbulence in the plasma maintains a dynamo, which amplifies the magnetic field in the jet and lobes. If so, this will involve the same processes that operate to maintain the magnetic field in the Sun and in most of the planets in the solar system.

Most current models of radio jets assume the jets can be descibed in terms of directed fluid flow, similar to buoyant smoke plumes or to supersonic jet-engine exhausts. Numerical simulations, combined with analytic modeling, have had quite a bit of success in reproducing the morphology of the sources. This work suggests the high-power sources are driven by jets that are very supersonic and much less dense than than the medium into which they propagate. Lower-power sources are thought to be transonic or subsonic flows; turbulence develops in the flow, which entrains the surrounding gas and decelerates the flow.

Alternative models have been proposed for these jets, however. It has been suggested that the jets are magnetohydrodynamic rather than hydrodynamic phenomena. In these models, the jet morphology and propagation are controlled as much or more by magnetic and electric forces as by the inertia and turbulent stresses that govern fluid flow. In addition, it has been proposed that the jets are collisionless particle beams - where only electric and magnetic forces on charged particles are important in the jet dynamics.

At this point, there are more competing models of jets than there are ways to discriminate between them. The next major advance in this field might involve a more specific comparison of the data with the models, than has been done up to now, in order to verify or disprove some of the theories described previously. When this is accomplished, the jets should be important tools for studying other astrophysical problems. They will provide probes of their local environment, through their interaction with the extragalactic gas, and with the local gravitational potential. But, perhaps their most interesting use will be to study the central engine in active galactic nuclei; why do some (but not all) active nuclei produce these jets, and how do they do so?

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7. See also Active Galaxies and Quasistellar Objects, Superluminal Motion; Galaxies, Radio Emission; Jets, Theory of.