Clues on the nature of Compact Steep Spectrum radio sources from optical spectroscopy

New long-slit spectra are presented for a group of Compact-Steep-Spectrum (CSS) sources (i.e. powerful radio sources with sub-galactic size and steep spectral index at high radio frequencies) selected from the southern (δ < 10◦) 2-Jy sample of radio sources with redshift limit of z = 0.7. Their spectra are strong in emission lines (e.g. [O III]λ5007 and [O II]λ3727), as expected for such powerful radio sources. The line luminosities and ratios obtained for these compact sources are compared with those for the other sources of the southern 2Jy sample. We find that the optical spectra for the CSS sources show characteristics very similar to the extended sources of similar radio power and redshift. In particular, we find that the CSS sources follow the correlation between the radio power and [O III]λ5007 and [O II]λ3727 luminosities found for the extended sources, although there is tentative evidence that they have lower [O III]λ5007 luminosities. Also, the optical continuum spectral energy distribution (SED) and polarization properties of the CSS sources show the same variety of characteristics observed among the extended sources. The first-order similarity that we find between the spectra of CSS sources and the extended sources of similar radio power is consistent with the idea that the CSS sources are young radio sources in which the radio jets have yet to emerge from the central regions of the host galaxies.