5 Conclusions

A sample of GPS sources has been selected from the Westerbork Northern Sky Survey, with flux densities one to two orders of magnitude lower than bright GPS sources investigated in earlier studies. Sources with inverted spectra at frequencies >325 MHz have been observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to select genuine GPS sources. This has resulted in a sample of 47 GPS sources with peak frequencies ranging from $\sim$500 MHz to >15 GHz, and peak flux densities ranging from $\sim$40 to $\sim$900 mJy.

Five GPS sources in our sample show extended emission or nearby components in the NVSS maps at 1.4 GHz. However it is not clear if these components are related to the GPS sources.

About 30% of the objects show flux density differences greater than 20% between the Greenbank and MERLIN 5 GHz measurements, with the Greenbank data points all higher than the MERLIN observations. We believe this is due to variability, and that the lack of sources with reverse variability (the MERLIN flux density greater than the Greenbank flux density) is due to a selection effect caused by the "old'' epoch (1987) of the Greenbank observations.

GPS source counts are comparable to 1/250 of the 2 GHz source counts for large scale radio sources, if the latter sources were to have 10 times their measured flux densities. Unfortunately, apparent differences in redshift distributions between GPS and large scale radio sources hamper a direct and straightforward interpretation of the source counts. Potentially, the comparison of GPS source counts with that of large scale radio sources can provide clues about the age of GPS sources and their luminosity evolution. If it is assumed that the redshift distributions are the same for GPS and large size radio sources, the source counts indicate that GPS sources have to decrease in luminosity by a factor of $\sim\!\! 10$ if they all evolve into large scale radio sources.

Acknowledgements

This research was partly supported by the European Commission, TMR Programme, Research Network Contract ERBFMRXCT96-0034 "CERES''.