1998A&A...333...70S

A relativistic radio component, which moves in a direction close to the sky plane, will increase in flux density when it decelerates. This effect is the basis for the qualitative model for GPS galaxies we present in this paper, which can explain their low-variability convex spectrum, their compact double or compact symmetric morphology, and the lack of GPS quasars at similar redshifts. Components are expelled from the nucleus at relativistic speeds at a large angle to the line of sight, and are decelerated (eg. by ram-pressure or entrainment of the external gas) before contributing to a mini-lobe. The young components are Doppler boosted in the direction of motion but appear fainter for the observer. The non-relativistic mini-lobes dominate the structure and are responsible for the low variability in flux density and the convex radio spectrum as well as the compact double angular morphology. Had the same source been orientated at a small angle to the line of sight, the young components would be boosted in the observer's direction resulting in a flat and variable radio spectrum at high frequencies. Hence the characteristic convex spectrum of a GPS source would not be seen. These sources at small angles to the line of sight are probably identified with quasars, and are not recognized as GPS sources, but are embedded in the large population of flat spectrum variable quasars and BL Lac objects. This leads to a deficiency in GPS quasars at z<1, but the model does not explain the population of GPS quasars at high redshift. However, there is increasing evidence that the high redshift GPS quasars are not related to the GPS galaxies. It is interesting for unification purposes to investigate if deceleration is a general phenomenon in compact radio sources. We have therefore investigated if evidence for deceleration can be traced in flux density outbursts of highly variable radio sources, and have found that indeed the evolution of the spectral peak in such outbursts does suggest that deceleration may play an important role, at least in some of these sources.