Gigahertz-peaked spectrum (GPS) sources are compact radio sources located in centres of distant active galaxies. The shape of the radio continuum spectrum of GPS sources is convex, the flux density increases towards high frequencies and decreases above a turnover at gigahertz-frequencies. Whereas the majority of extragalactic radio sources extend well outside of their host galaxies, these compact sources reside only in the central regions of their host. For this reason studying GPS sources can provide us with information on the structure and the properties of active galactic nuclei.
In this thesis, the radio spectra and the total flux density variability of GPS sources are studied. We have collected an extensive database of radio observations of GPS sources and candidates from the monitoring programmes of Metsähovi Radio Observatory and University of Michigan Radio Astronomy Observatory, and from the literature. We have also made new observations of the sources. In the literature, the GPS classification is often done using few non-simultaneous data points presuming that the sources are not variable. Our results show that this approach has produced samples that are heavily contaminated by misclassified sources. Among the quasar-type GPS sources classified in the literature, there is only a small fraction of genuine GPS sources as highly variable sources with temporary GPS features in their radio spectrum have been misclassified as GPS sources. The fraction of genuine GPS galaxies was found to be larger but the contamination was significant also in galaxy-type samples. In addition, the genuine GPS sources were found to be variable contrary to earlier conception.
Cluster analyses of GPS sources presented in this thesis also support the view of heterogeneous GPS samples. The blazar-type sources with temporarily gigahertz-peaked spectra clearly stood out as their own cluster, and the sources with confirmed GPS-type spectra formed several different clusters. This result supports the view that there are different types of genuine GPS sources
