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 c
atalog: 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.
We present new polarisation observations at 13 cm, acquired using the Westerbork Synthesis Radio Telescope (WSRT), of 65 sources, from the B3-VLA sample of Compact Steep-Spectrum sources. These new data are combined with our VLA polarisation data, at
3.6, 6 and, 21 cm, presented in a previous paper. Due to the multi-channel frequency capabilities of the WSRT, these new 13 cm observations enable a more reliable determination of integrated Rotation Measures, and of depolarisation behaviour with wavelength. The new data are inconsistent with the depolarisation models that we used earlier, and we propose an alternative model which seems to work properly. We also revise our previous model for the external Faraday screen, and its dependence on the source redshift.
