  Compact steep-spectrum radio sources and giga-hertz peaked spectrum radio
sources (CSS/GPS) are generally considered to be mostly young radio sources. In
recent years we studied at many wavelengths a sample of these objects selected
from the B3-VLA catalog: the B3-VLA CSS sample. Only ~ 60 % of the sources were
optically identified. We aim to increase the number of optical identifications
and study the properties of the host galaxies of young radio sources. We
cross-correlated the CSS B3-VLA sample with the Sloan Digital Sky Survey
(SDSS), DR7, and complemented the SDSS photometry with available GALEX (DR 4/5
and 6) and near-IR data from UKIRT and 2MASS. We obtained new identifications
and photometric redshifts for eight faint galaxies and for one quasar and two
quasar candidates. Overall we have 27 galaxies with SDSS photometry in five
bands, for which we derived the ultraviolet-optical spectral energy
distribution (UV-O-SED). We extended our investigation to additional CSS/GPS
selected from the literature. Most of the galaxies show an excess of
ultra-violet (UV) radiation compared with the UV-O-SED of local radio-quiet
ellipticals. We found a strong dependence of the UV excess on redshift and
analyzed it assuming that it is generated either from the nucleus (hidden
quasar) or from a young stellar population (YSP). We also compare the UV-O-SEDs
of our CSS/GPS sources with those of a selection of large size (LSO) powerful
radio sources from the literature. If the major process of the UV excess is
caused by a YSP, our conclusion is that it is the result of the merger process
that also triggered the onset of the radio source with some time delay. We do
not see evidence for a major contribution from a YSP triggered by the radio
sources itself.
