  A sample of 47 faint Gigahertz Peaked Spectrum (GPS) radio sources selected
from the Westerbork Northern Sky Survey (WENSS, Rengelink et al. 1997), has
been imaged in the optical and near infrared, resulting in an identification
fraction of 87%. The R-I and R-K colours of the faint optical counterparts are
as expected for passively evolving elliptical galaxies, assuming that they
follow the R band Hubble diagram as determined for radio-bright GPS galaxies.
We have found evidence that the radio spectral properties of the GPS quasars
are different from those of GPS galaxies: The observed distribution of radio
spectral peak frequencies for GPS sources optically identified with bright
stellar objects (presumably quasars) is shifted compared with GPS sources
identified with faint or extended optical objects (presumably galaxies), in the
sense that a GPS quasar is likely to have a higher peak frequency than a GPS
galaxy. This means that the true peak frequency distribution is different for
the GPS galaxies and quasars, because the sample selection effects are
independent of optical identification. The correlation between peak frequency
and redshift as has been suggested for bright sources has not been found in
this sample; no correlation exists between R magnitude (and therefore redshift)
and peak frequency for the GPS galaxies. We therefore believe that the claimed
correlation is actually caused by the dependence of the peak frequency on
optical host, because the GPS galaxies are in general at lower redshifts than
the quasars. The difference in the peak frequency distributions of the GPS
galaxies and quasars is further evidence against the hypothesis that they form
a single class of object.
