Annu. Rev. Astron. Astrophys. 1981. 19: 373-410
Copyright © 1981 by . All rights reserved

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Perhaps the most significant result to come from the study of compact radio sources is the growing acceptance that bulk relativistic motion may be common, and that it provides a natural interpretation for

(a) superluminal component motion;

(b) rapid flux density variations;

(c) absence of inverse Compton scattered X rays during outbursts;

(d) absence of interstellar scintillations in rapid flux variables;

(e) the efficient transfer of energy from its source to the extended radio lobes which may be up to a megaparsec or more away.

The observation, based on the lack of depolarization, that the compact sources are not contained by thermal plasma, and the direct determination of motions in the Galactic object SS 433 with v ~ 0.3 c (Abell & Margon 1979) both contribute to the acceptance of possible motions on a galactic scale with v ~ c.

Non-cosmological red shifts, which have sometimes been suggested as the single simplifying assumption necessary to explain all of the above observations, appear even less attractive now with the detection of a stellar content in 3C 120 (Baldwin et al. 1980), BL Lac (Miller et al. 1978), and 3C 273 (Wyckoff et al. 1980) which would appear to remove any doubt as to their distance.

Many of the problems we have discussed here for the compact radio sources involving high luminosities from apparently small volumes as well as the rapid flux variations which appear to violate inverse Compton limits, may be even more critical in the large optical and infrared wavelength outbursts (Rieke et al. 1976, Stein et al. 1976, Rieke & Lebofsky 1979, Angel & Stockman 1980).

Relativistic beaming may also be important in interpreting the compact optical and infrared sources, particularly the very bright outbursts sometimes observed in BL Lac objects. Indeed the so-called BL Lac phenomenon may be just those objects in which the optical emission is beamed toward the observer, so that the emission-line radiation is swamped by the Doppler-enhanced continuum emission. It is unlikely that this effect can be important in normal quasars, as the line and continuum intensities are typically comparable, and there is no evidence that the line emission in quasars contains a significant blue shift.

Guthrie & Napier (1975) and S. van den Bergh (1978, private communication) have suggested that the one-sided optical continuum and radio jets in M87 and 3C 273 may also be the blue-shifted components of a symmetric ejection. We note, in addition, that in some superluminal sources, asymmetric extended radio emission is sometimes found along the line of motion, but tens of kiloparsecs (Davis et al. 1978, Perley & Johnston 1979), or even several megaparsecs (Reich et al. 1980) away, suggesting that bulk relativistic motion may be important at much larger distances from the parent object than previously supposed.

It is interesting to note that the concept of relativistic beaming was first introduced as early as 1963 by Shklovsky (1963, 1965) as a way to explain both the asymmetric appearance of the jets in M87 and 3C 273, to reduce the apparently large energy densities required from isotropic models, and to eliminate the problem of short lifetimes of electrons responsible for the optical synchrotron emission. The growing evidence for relativistic motion in compact radio sources makes it possible to understand a wide range of otherwise perplexing observations without the need to invoke exotic physics or cosmologies. Future observations of the structure of the compact quasars and galactic nuclei made as a function of frequency and time with high resolution arrays will surely lead to a better understanding of these objects, and perhaps to the ultimate source of energy, in radio galaxies and quasars as well.


We are grateful to the many colleagues who provided data and preprints in advance of publication, and particularly to the Caltech group who generously made available some of their most recent results. P. Biermann, M. Cohen, R. Porcas, E. Preuss, A. Readhead, L. Rudnick, R. Schilizzi, and D. Shaffer made valuable comments which have helped to improve the manuscript. Part of this work was done while KIK was on leave at the Max-Planck-Institut fur Radioastronomie.

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