Leiden Observatory, P.O. Box 9513, 2300 RA, Leiden, The Netherlands
2 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
3 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA, Dwingeloo The Netherlands
4 Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
5 Kapteyn Institute, Postbus 800, 9700 AV, Groningen, The Netherlands
6 Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France
Accepted: 18 March 1998
The Westerbork Northern Sky Survey (WENSS) has been used to select a sample of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two orders of magnitude lower than bright GPS sources investigated in earlier studies. Sources with inverted spectra at frequencies above 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 ~500 MHz to GHz, and peak flux densities ranging from ~40 to ~900 mJy. Counts of GPS sources in our sample as a function of flux density have been compared with counts of large scale sources from WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The counts can be made similar if the number of large scale sources at 2 GHz is divided by 250, and their flux densities increase by a factor of 10. On the scenario that all GPS sources evolve into large scale radio sources, these results show that the lifetime of a typical GPS source is ~250 times shorter than a typical large scale radio source, and that the source luminosity must decrease by a factor of ~10 in evolving from GPS to large scale radio source. However, we note that the redshift distributions of GPS and large scale radio sources are different and that this hampers a direct and straightforward interpretation of the source counts. Further modeling of radio source evolution combined with cosmological evolution of the radio luminosity function for large sources is required.
Key words: galaxies: active / quasars / radio continuum: galaxies
The Westerbork Synthesis Radio telescope (WSRT) is operated by the Netherlands Foundation for Research in Astronomy with financial support from the Netherlands Organisation for Scientific Research (NWO).
© European Southern Observatory (ESO), 1998