Compact Steep-Spectrum (CSS) and GHz-Peaked Spectrum (GPS) radio sources are intrinsically small objects (Fanti et al. 1990) with high frequency steep spectra ( with ), projected linear size < 15 kpc and very low linear polarization with a few outstanding exceptions. High frequency radio flux density variability is absent among CSS-GPS galaxies, while it might play a role for quasars, although prominent outbursts have never been observed (Stanghellini et al. 1997).
These properties have been interpreted in terms of youth (Phillips & Mutel 1982; Carvalho 1985; Mutel & Phillips 1988; Fanti et al. 1995; Readhead et al. 1996; O'Dea & Baum 1997), where the jets of a recently triggered radio activity are still digging their way into the interstellar medium. In this scenario, GPS and CSS sources represent an early-intermediate stage of the radio source life not yet developed into the classical extended double FRII sources (following the nomenclature by Readhead et al. 1996, the Compact Symmetric Objects (CSO) often found among GPS galaxies will evolve into Medium Symmetric Objects (MSO) common among CSS objects, and finally into extended FRII radio galaxies, as the source grows and gets older, diminishing its luminosity by about one order of magnitude).
Alternatively, they could be "frustrated'' sources, with anomalously dense nuclear environments impeding the source expansion (van Breugel et al. 1984). Relic extended radio emission is found in about 1/5 of GPS radio sources (Baum et al. 1990; Stanghellini et al. 1990). Baum et al. (1990) interpret this as evidence for smothering produced by infalling gas and dust; this is consistent with the observation of "disturbed'' optical images, indication of possible merger activity, often seen in the GPS source hosts (Stanghellini et al. 1993; O'Dea et al. 1996), and with the presence of relic radio emission seen in about 10-20% of GPS sources (Stanghellini et al. in preparation). An outcome from this scenario is that the radio emitting electrons are confined to the nuclear region by the increased ambient density induced by the merger process.
GPS and CSS sources represent a significant fraction of flux limited catalogues (15-30%, depending on the selection frequency where the higher fraction is found at higher frequency). They are found at moderate or high redshifts (generally z>0.2 but many objects have z>1.5) and it is remarkable that there does not exist a counterpart of such powerful and compact radio sources in the nearby objects population.
The radio spectra of these objects show a turnover occurring at tens or a few hundreds of MHz (CSS) or at higher frequencies (GPS) and this phenomenon is generally interpreted in terms of synchrotron self-absorption. A direct implication is that generally GPS are smaller than CSS sources.
The samples studied so far (Spencer et al. 1989; Fanti et al. 1990; O'Dea et al. 1990; Dallacasa et al. 1995; Stanghellini et al. 1997) were limited to the most powerful objects, drawn from catalogues at various frequencies. Here we present new result on sources having radio power comparable to the objects in the forementioned papers, but found from other works, not specifically related to the CSS/GPS field.
Furthermore, objects with lower power will help in understanding the relation between the CSS-GPS objects and the other classes of radio sources and possibly with the much weaker nearby CSS-GPS's. Work in this direction is carried out by Snellen et al. (1995)
Section 2 describes the VLBI observations and the data reduction, Sect. 3 briefly presents the sources observed here; in Sect. 4, each source is analyzed, and Sect. 5 summarizes the results.
In this paper we use H0=100 km s-1 Mpc-1 and q0=0.5.