General introduction

History

In a description of an optical image of M 87 (NGC 4486), Curtis (1918) wrote “a curious straight ray … connected with the Nucleus”. By the 1950s the term ‘jet’ was being used to describe this feature which it seemed plausible to associate with ejection of material from the innermost region of the galaxy (Baade & Minkowski 1954), although the concept of a continuous flow was not then envisaged. Baade (1956) measured the optical polarization of the M 87 jet, supporting the idea that the material was a synchrotron emitting plasma akin to that of the Crab supernova remnant.

Shklovskii (1963), in an attempt to explain the double radio sources and M 87's jet, discussed many ideas that play a role in current theories – accretion of matter in the gravitational potential of a galactic nucleus; the consequent heating of a plasma that breaks out along a preferred axis; the flow of this material into intergalactic space and the re-energization of the electrons within the flow. However, the model still did not encompass the idea of a continuous flow, carrying energy in the form of bulk motion. Schmidt (1963) wrote of “a wisp or jet” on the image of the optical counterpart to 3C 273, and by about this time, the term ‘jet’ was in common usage (e.g., Greenstein & Schmidt 1965; Burbidge, Burbidge & Sandage 1965) – but still without a clear recognition that a continuous flow of matter and energy was involved.