Use of the term "jet" in extragalactic astronomy dates back to Baade & Minkowski (1954), who described an optical feature in M87 as "a unique peculiarity known for a long time ... a straight jet extending from the nucleus in p.a. 290°, bluer than the nebula itself ... several strong condensations". This feature was first recorded by Curtis (1918), as "a curious straight ray apparently connected with the nucleus by a thin line of matter". Its linear polarization at optical wavelengths (Baade 1956) provided early evidence for synchrotron emission from extragalactic radio sources.
The existence of radio emission from AGN jets was also known for many years before the VLA went into operation.
The first sign of radio emission from a "jet" had come via Schmidt's (1963) identification of "a star of about thirteenth magnitude and a faint wisp or jet" near the accurate positions of the radio components of 3C273 (Hazard et al. 1963). Schmidt's identification of radio component `B' with the "star" marks the start of the quasar industry. His identification of component `A' with the tip of the "faint wisp or jet" likewise marks the start of a radio jet industry that developed much more slowly. Six years passed before Hogg et al. (1969) showed that a compact extranuclear radio component in 3C274 coincided with the brightest knot in M87's optical jet (thus providing evidence for radio emission from, rather than just around, that optical feature).
The next evidence for radio jets came from observations made at Cambridge. Northover (1973) found a "narrow jet of emission linking the galactic nucleus to one of the extended regions" in the low-power plume-like radio source 3C66B, and suggested that this implied a continuous resupply of energy from the nucleus to the extended source (in compact sub-components that he interpreted as buoyant "bubbles"). Hargrave & Ryle (1974) used high-resolution imaging of Cygnus A's hot spots and spectral-aging analysis to argue for "continuous replenishment of energetic electrons within the two main compact components", favoring models with "continuing ejection of beams of energetic particles or low frequency waves from the nucleus of the galaxy". The first direct evidence for a radio jet in a powerful "classical double" source came when Turland (1975) detected the abbreviated jet in the radio galaxy 3C219. A spectacularly long and narrow one-sided jet was found in the giant radio galaxy NGC6251 by Waggett et al. (1977).
Radio jets were also recognized in WSRT images of several radio galaxies at about this time: van Breugel & Miley (1977) reported jets in B0844+319 and 3C129, and gave retrospective evidence for jets in several other galaxies, including 3C449 (Högbom & Carlsson 1974) and 3C83.1 (Miley et al. 1975).
Alan Bridle
Tue Oct 13 15:58:42 EDT 1998