1 Introduction

Gigahertz Peaked Spectrum (GPS) radio sources are a class of extragalactic radio source characterized by a spectral peak near 1 Gigahertz in frequency (e.g. Spoelstra et al. 1985). The spectral peak in these compact luminous objects is believed to be due to synchrotron self absorption caused by the high density of the synchrotron emitting electrons in the radio source. GPS sources are interesting objects, both as Active Galactic Nuclei (AGN) and as cosmological probes. It has been suggested that they are young radio sources (<10⁴ yr) which evolve into large radio sources (Fanti et al. 1995; Readhead et al. 1996; O'Dea & Baum 1997), and studying them would then provide us with important information on the early stages of radio source evolution. Alternatively GPS sources may be compact because a particularly dense environment prevents them from growing larger (e.g. O'Dea et al. 1991).

Important information about the nature of GPS radio sources comes from the properties of their optical counterparts. The galaxies appear to be a homogeneous class of giant ellipticals with old stellar populations (Snellen et al. 1996a,b; O'Dea et al. 1996) and are thus useful probes of galaxy evolution with little or no contamination from the active nucleus in the optical. GPS quasars have a different redshift distribution to their galaxy counterparts (2<z<4, O'Dea et al. 1991) and their radio morphologies are also quite unlike those of GPS galaxies. The relationship between GPS quasars and galaxies, if any, remains uncertain (Stanghellini et al. 1996).

Previous work on GPS sources has concentrated on the radio-bright members of the class, with $S_{\rm 5GHz}\gt 1$ Jy (Fanti et al. 1990; O'Dea et al. 1991; Stanghellini et al. 1996; de Vries et al. 1997). We are carrying out investigations of GPS sources at fainter flux density levels, in order to compare their properties with their radio bright counterparts. This enables us to investigate the properties of GPS sources as a function of radio luminosity, redshift, and rest frame peak frequency. The selection of a sample at intermediate flux densities was described in Snellen et al. (1995a). This paper describes and discusses the selection of an even fainter sample from the Westerbork Northern Sky Survey (WENSS, Rengelink et al. 1997).