Optical spectroscopy of faint gigahertz peaked-spectrum sources

We present spectroscopic observations of a sample of faint gigahertz peaked -spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey (WENSS), Redshifts have been determined for 19 (40 per cent) of the object s. The optical spectra of the GPS sources identified with low-redshift gala xies show deep stellar absorption features. This confirms previous suggesti ons that their optical light is not significantly contaminated by active ga lactic nucleus-related emission, but is dominated by a population of old (> 9 Gyr) and metal-rich (>0.2 [Fe/H]) stars, justifying the use of these (pro bably) young radio sources as probes of galaxy evolution. The optical spect ra of GPS sources identified with quasars are indistinguishable from those of flat-spectrum quasars, and clearly different from the spectra of compact steep-spectrum (CSS) quasars. The redshift distribution of the GPS quasars in our radio-faint sample is comparable to that of the bright samples pres ented in the literature, peaking at z similar to 2-3, It is unlikely that a significant population of low-redshift GPS quasars is missed as a result o f selection effects in our sample. We therefore claim that there is a genui ne difference between the redshift distributions of GPS galaxies and quasar s, which, because it is present in both the radio-faint and bright samples, cannot be caused by a redshift-luminosity degeneracy. It is therefore unli kely that the GPS quasars and galaxies are unified by orientation, unless t he quasar opening angle is a strong function of redshift. We suggest that t he GPS quasars and galaxies are unrelated populations and just happen to ha ve identical observed radio spectral properties, and hypothesize that GPS q uasars are a subclass of flat-spectrum quasars.