1.1. The Early History of the Study of CSS and GPS Sources

The existence of GPS and CSS sources has been known since the early days of extragalactic radio astronomy. Examples from the 3C sample of compact ( 1") radio sources with steep spectra that turn over at frequencies around 100 MHz were discovered in the early 1960s (Conway, Kellermann, & Long 1963; Allen et al. 1962). These were explained as being due to synchrotron self-absorption in sources of angular size 0".1 (Williams 1963; Slish 1963). In 1963, Bolton, Gardener, & Mackey reported the discovery of 1934-63, the archetype GPS source. The spectrum of 1934-63 was discussed in detail by Kellermann (1966a) in terms of synchrotron self-absorption, free-free absorption, and a signal from an extraterrestrial civilization.

The study of GPS and CSS sources began in earnest in the early 1980s. Evidence accumulated that the CSS sources made up a significant fraction (~ 30%) of sources selected at 2.7 or 5.0 GHz (Kapahi 1981; Peacock & Wall 1982; and cf. Gopal-Krishna, Steppe, & Witzel 1980; Gopal-Krishna, Preuss, & Schilizzi 1980). Phillips & Mutel (1982) called attention to the symmetric compact double structures in GPS sources, and Gopal-Krishna, Patnaik, & Steppe (1983) published the first large list of GPS sources.