Most strong, compact () radio sources when imaged at high resolution have a core-jet morphology, consisting of a bright unresolved core and a one-sided jet in which superluminal motion is often observed. These core-jet sources are thought to be due to the bases of relativistically beamed jets orientated close to the line of sight. However Phillips & Mutel (1982) first identified compact objects which appeared to be dominated by two unbeamed emission components and called them `Compact Doubles'. Conway et al. (1992) described two similar objects with more complex triple structures. Given this wider range of morpholgies Wilkinson et al. (1994) renamed this whole class of radio sources as `Compact Symmetric Objects' (CSOs) emphasising their primary property of symmetry. Characteristically these objects show high luminosity radio emission regions separated by kpc which are located symmetrically on both sides of the centre of activity. It is thought that these high brightness regions are due to hotspots and minilobes created by the termination of oppositely directed jets and that this emission is free from relativistic beaming effects. For a recent review of the properties of CSOs and related sources see O'Dea (1998).
From the earliest papers it was suggested that CSOs were young sources (Phillips & Mutel 1982), which evolved into larger-sized objects. Alternatively it has been proposed that CSOs are `frustrated' sources, in which higher density and/or turbulence in the interstellar medium inhibits their growth to larger dimensions (van Breugel et al. 1984). Finally it has been proposed that they are a separate class of short lived objects, which `fizzle out' after about 104 yrs and do not grow to large sized objects (Readhead et al. 1994).
Detailed theories of the youth model of compact sources show that it is feasible that CSOs are part of an evolutionary sequence in which they later evolve into the slightly larger Compact Steep Spectrum (CSS) sources, which finally evolve into classical doubles (Fanti et al. 1995, Readhead et al. 1996a,b). De Young (1996) and Begelman (1996) have used simple physical models to confirm that CSO sources are probably not frustrated and confined but instead evolving. An obvious way to distinguish between competing models is to try to measure or set limits on the growth in overall size of CSOs and so determine their ages directly, which is the purpose of this paper.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998