8.2.4 Compact Steep-Spectrum and Gigahertz Peaked-Spectrum Sources

The place of Compact Steep-Spectrum Sources (CSS) and the possibly related Gigahertz Peaked-Spectrum Sources (GPS) in the unified scheme for high-power radio sources is an open question. CSS and GPS include both quasars and radio galaxies. Their most striking feature is their compact radio structure ( 15 and 1 kpc, respectively; see Fanti et al. 1990a and O'Dea et al. 1991 for reviews of their properties). In terms of the definitions in Sec. 1 (Table 1), we include them in the SSRQ/FSRQ and NLRG classes, as appropriate.

The CSS and GPS sources constitute a non-negligible part of radio samples; for example, CSS represent approximately 13% of the 3CR sample (Fanti et al. 1990a) and ~ 20% of the 2 Jy sample (Morganti et al. 1993). For our statistical analysis in Sec. 6, we removed the galaxies (because they were not classified as either FR I or FR II by their radio morphologies) but included the quasars; this might have increased the critical angles slightly.

Statistical considerations exclude the possibility that most CSS and GPS are intrinsically large radio sources seen at small angles to the line of sight. Instead, they are probably inherently compact (Fanti et al. 1990b). At the same time, the size distribution and relative numbers of CSS quasars and galaxies are consistent with the former being the latter viewed at angles smaller than ~ 45° (Fanti et al. 1990b), although the different radio morphologies of the two classes are not entirely explained by orientation. CSS and GPS could represent an early stage of radio source evolution (Fanti et al. 1990b) and if so, probably should be included in unification. Alternatively, they may lie in unusual environments (O'Dea et al. 1991). In either case, unified schemes bear on their origin, and it is important to understand how they fit into the present paradigm.