1. INTRODUCTION

Cygnus A was among the first discrete cosmic radio sources identified, using Yagi antennas, parabolic dishes, and the `sea interferometer' (Hey, Phillips, and Parsons 1946, Hey, Parsons and Phillips 1946, Bolton and Stanley 1948, Bolton 1948; see also Bolton 1982, Slee 1994). Early interferometric observations revealed Cygnus A as a double radio source (Jennison and Das Gupta 1953), and allowed for an accurate position to be determined (Smith 1951, Mills 1952, Hanbury-Brown, Jennison, and Das Gupta 1952; see also Smith 1984). Following earlier work by Mills and Thomas (1951) including an attempt to measure the annual parallax, the accurate position was used by Dewhirst (1951) and Baade and Minkowski (1954) in their optical identification of Cygnus A as the first ultra-luminous active galaxy. The enormous energy involved was initially explained within a colliding galaxy scenario (Baade and Minkowski 1954, Minkowski and Greenstein 1954), but the double nature of the radio source, stretching outside the optical object, was puzzling.

Since its discovery, Cygnus A has played a fundamental role in the development of jet theory for powering the double structures in powerful radio galaxies. Early aperture synthesis images of Cygnus A led to the discovery of radio `hotspots' at the source extremities (Mitton and Ryle 1969, Hargrave and Ryle 1974). As first pointed out by Hargrave and Ryle (1974), the radiative lifetimes of the relativistic electrons in these hotspots ( few x 10<sup>4</sup> years) are less than the light travel time from the nucleus to the hotspot ( 1.5 x 10<sup>5</sup> yrs, for H<sub>0</sub> = 100 km sec<sup>-1</sup> Mpc<sup>-1</sup>), hence requiring continuous injection of a population of relativistic electrons at the hotspots. This conclusion led to the `beam' or `jet' model for powering double radio sources (Blandford and Rees 1974, Scheuer 1974). The jet model also avoids the dramatic expansion losses inherent in single-burst `plasmon' models for extended structures in radio galaxies (van der Laan and Perola 1969, Longair, Ryle, and Scheuer 1973, DeYoung and Axford 1967). The Cygnus A jet itself was suggested in early VLBI observations of the nucleus (Kellermann et al. 1975, Kellermann et al. 1981, Linfield 1982), and finally revealed in detail by the first high dynamic range images of the source with the VLA (Perley, Dreher, and Cowan 1984).

More generally, Cygnus A has played a key role in the field of radio-loud active galaxy research (e.g., Burbidge, Burbidge and Sandage 1963). It was among the first such galaxies to be detected, and is by far the closest of the ultra-powerful radio galaxies. Fig. 1 shows the classic `Malmquist bias' in the flux limited 3C sample of powerful radio galaxies (Laing et al. 1983, from Stockton and Ridgeway 1996). These are all high luminosity (spectral luminosity 10<sup>33</sup> ergs sec<sup>-1</sup> Hz<sup>-1</sup> for H<sub>0</sub> = 75 km sec<sup>-1</sup> Mpc<sup>-1</sup> at rest frequency 178 MHz), edge-brightened, Fanaroff-Riley class II radio galaxies (FRII or `classical doubles'; Fanaroff and Riley 1974) ⁽¹⁾.

Figure 1. The radio power-redshift relation for the 3C sample of z < 1 FRII radio galaxies, calculated using H0 = 75 km sec-1 Mpc-1 and q0 = 0.5 (from Stockton and Ridgeway 1996). The more luminous sources are labeled.

The position of Cygnus A, 3C 405, on this redshift-luminosity diagram is unique: a very high radio luminosity - more than two orders of magnitude above the standard FRI-FRII break, with a very low redshift (z = 0.0562, Stockton et al. 1994). Cygnus A is about 1.5 orders of magnitude more luminous than any other source at z 0.1. Indeed the next closest source of similar luminosity is 3C 295, at z = 0.46, and most sources with luminosities similar to Cygnus A are located at z 1. Hence, Cygnus A may provide an example to separate the effects of radio luminosity and epoch in the currently topical field of high redshift radio galaxies (McCarthy 1993).

Its enormous radio flux density has allowed for high S/N radio observations from the early days of radio astronomy. This is in contrast to the optical and infrared where high S/N observations of the faint galaxy require sensitive detectors and large apertures, and progress was slower. However, renewed interest in the optical and near infrared properties arose with the proposition that the Cygnus A galaxy might harbour a QSO in its nucleus (Pierce and Stockton 1986, Tadhunter et al. 1990, Vestergaard and Barthel 1993).

Following are some conventions used in this review. We use h = H₀ / 100 km sec^-1 Mpc^-1 and q₀ = 0.5, implying a luminosity distance for Cygnus A of 172/h Mpc, and an angular size distance of 154/h Mpc such that 1" = 0.75 h^-1 kpc. Spectral index, , is defined as a function of frequency, , and intensity, I(), as: I() . We use cgs units unless stated otherwise. The symbol `c' is used for the speed of light.

Some basic observational parameters for Cygnus A are listed in Table 1. This review is organized by physical question, starting with the radio source. We begin with a short review of jet theory for powerful radio galaxies. We then discuss the radio continuum morphology and spectra of the jets, hotspots, and lobes, followed by a review of internal and foreground magnetic fields (as inferred from the radio data). The subsequent section discusses the X-ray cluster and the interaction between the radio source and the cluster gas. We then summarize the optical and infrared properties of the Cygnus A galaxy and discuss in detail the active nuclear regions, including the question of a possible hidden quasar in Cygnus A. We present an overview of models of central engines in AGN and observations of Cygnus A which may be relevant to testing such models. We conclude with a brief section concerning the question of whether Cygnus A is representative of powerful high redshift radio galaxies. Our literature search is complete to mid 1995, and the review includes views and results presented at the May 1995 Cygnus A Workshop held at NRAO, Greenbank, WV (see Carilli and Harris 1996).

¹ For comparison, radio galaxies of lower luminosity typically show edge-darkened morphologies, and are called FRI sources. Back.