After almost two decades of work on the GPS and CSS sources we have obtained a much clearer picture of what their essential properties are. However, there is still controversy about their origin and relationship to the large-scale powerful sources. In this section I give my own perspective on these enigmatic sources.
The idea that they are some kind of exotic class of radio source unrelated to the large sources (i.e., transient sources) is probably not completely ruled out. But in my opinion, the similarities of many of their global properties, including optical host galaxies, emission-line properties, and near-IR properties, to those of the large sources suggest that they are similar types of AGNs in similar types of galaxies and are indeed related to the large sources. This is not to say that the GPS/CSS sources do not hold a few surprises for us.
It seems clear that the GPS/CSS radio sources do interact with their ambient medium, and much more strongly than do large sources at low and intermediate redshifts. However, at this point, the "smoking gun" of large gas masses ( 1010 M) in the host that would imply confinement/frustration has not been found. However, only a handful of objects have been searched carefully (section 10). As we all know, absence of evidence does not mean evidence of absence. So it is important to conduct deep searches for gas in a significant number of GPS and CSS sources. Work in progress with the new low-frequency receivers at WSRT should help answer this question.
At the present time, the hypothesis that is not ruled out by any data is that the GPS and CSS sources are young versions of the large radio sources. The fact that many of the global properties of GPS/CSS sources are similar to those of the large sources supports this. In addition, the striking resemblance of the radio morphologies, especially those of the CSOs, also argues for an evolutionary relationship.
If true, this is very exciting, since we will finally be able to use samples of GPS/CSS in combination with samples of large sources to place constraints on the evolution of radio sources. The current analysis suggests a significant decline in radio luminosity as the sources evolve. Thus, the GPS and CSS in the bright samples studied so far will evolve into intermediate luminosity radio sources. If this is the case, the progenitors of the LRL classical doubles have not been found. This may be because they are so rare, i.e., we expect 0.1% of the population to be visible in the GPS state at any time, and/or because the very powerful sources have radio spectra that peak above 5 GHz when they are young.