Young Extragalactic Radio Sources

Gigahertz Peaked Spectrum (GPS) sources and Compact Symmetric Objects (CSO) are selected in very different ways, but have a significant overlap in properties. Ever since their discovery it has been speculated that they are young objects, but only recently, strong evidence has been provided indicating that GPS sources and CSOs are indeed the young counterparts of large, extended sources. They are therefore the objects of choice to study the initial evolution of extragalactic radio sources. Observational constraints on the luminosity evolution of young radio sources have mainly come from number density statistics and source size distributions, indicating that young sources should decrease in luminosity by a factor ∼ 10 as they evolve to extended objects. We argue that the growth or decay in radio power of the individual objects has a strong influence on the slope of their collective luminosity function. This has led to a new method of constraining the evolution of young sources by comparing their luminosity function to that of large extended objects. The luminosity function of GPS sources is shown to be flatter than that of extended objects. This is consistent with the proposed scenario that young sources increase in radio luminosity and large size radio sources decrease in luminosity with time, but larger homogeneous samples are needed. The new large area radio surveys and the current sensitivity and flexibility of VLBI networks allow the construction and investigation of such large and homogeneous samples of young sources.