11.3. Young, Evolving Sources
The idea that GPS sources are young was suggested by Shklovsky (1965) for the case of 1934-638 and by Blake (1970) for an early sample of GPS sources based on their peaked spectra and compact size. Readhead & Hewish (1976) suggested that if Scheuer's (1974) beam model is correct, the small 3CR sources should evolve into the larger ones. Phillips & Mutel (1982) pointed out the simple symmetric radio structure in four GPS radio galaxies (then called compact doubles) and suggested that they were young versions of the large classical doubles. Carvalho (1985) presented a simple analytic model (based on Scheuer 1974) for the evolution of GPS sources that guided much of the later discussion. Mutel and collaborators (Hodges & Mutel 1987; Mutel & Phillips 1988; Mutel, Su, & Song 1990) applied Carvalho's model to small samples of GPS, CSS, and large-scale doubles and suggested that these sources represented an evolutionary sequence. Begelman (1996) presented an analytic self-similar evolution model for CSOs that was adapted by Bicknell et al. (1997) to explain the relationship between the radio source and the emission-line nebula. Saikia et al. (1996) presented simulations of self-similar radio source models, while De Young (1996) has presented simulations of non-self-similar jet evolution, both of which seem to produce consistent evolution. Snellen et al. (1998) have presented a relativistic plasmon model for GPS sources.
The hypothesis that the GPS sources evolve through a CSS stage on their way to become large sources is in general consistent with observations of CSS sources (Fanti et al. 1990b, 1995), CSOs (Readhead 1995; Readhead et al. 1996a, 1996b), and combined samples of GPS and CSS sources (O'Dea & Baum 1997) and comparisons of these samples with large-scale powerful sources. Bremer, Fabian, & Crawford (1997) have suggested that radio sources in central cluster galaxies with cooling flows are GPS sources when young. However, the converse is not necessarily true, i.e., it is clear that not all GPS sources are in cooling flows (section 9).
The main arguments in favor of evolution are the following. (1) The GPS and CSS sources have morphologies similar to those of the large-scale sources (section 3). (2) Although there is gas in their environments, there is currently no compelling evidence that there is enough dense gas to confine the majority of the GPS and CSS sources (sections 10 and 11.2). (3) In the absence of confinement by cold gas, the internal pressure in GPS and CSS radio sources is sufficient to allow expansion in ram pressure balance with expected ISM densities at velocities of a few to 10% of the speed of light (Phillips & Mutel 1982; Mutel, Hodges, & Phillips 1985; Fanti et al. 1990b; Readhead 1995; Readhead et al. 1996b). (4) The GPS and CSS sources do not show evidence for a large halo of diffuse emission (i.e., a "wastebasket"), which is expected if they are confined for their lifetimes (Readhead et al. 1996b).
At the present time, the weight of the evidence seems to favor the young and evolving source model for GPS and CSS sources. However, the answer is not definitive, and in fact more than one explanation may apply to this population of sources.