1 Introduction

Among the many open questions about powerful radiogalaxies is the early evolution of these objects. This is an important issue not only from the point of view of the detailed phenomenology of these objects but also for understanding the evolution of massive galaxies. Galaxies in their early stage of radio activity are likely to have their nuclear regions enshrouded in a cocoon or thick disk of material left over from the event(s) that trigger the activity. The radio jet expanding in this medium will interact and sweep out this rich interstellar medium (ISM). These processes have been suggested to profoundly affect even the star formation history in luminous galaxies (Silk & Rees 1998). Thus, the study of the early-phase of radio activity and its effect on the galaxy medium has broad implications.

Compact Steep Spectrum (CSS) and Gigahertz Peaked Spectrum (GPS) radio sources have now been recognised to be the likely candidates of radio sources in such an early stage of their life. These radio sources are powerful ( $P_{2.7~{\rm GHz}} \raisebox{-0.6ex}{$~\stackrel {\raisebox{-.2ex}{$\textstyle >$ }}{\sim}~$ }10^{25.5}$ W Hz^-1), have a steep spectral index at high frequencies ( $\alpha < -0.5$, $S \sim \nu^{\alpha}$) and sub-galactic size (<20 kpc). Indeed, the turnover often observed in their spectra (around 1 GHz for the GPSs and about 100 MHz for the CSSs) is usually interpreted as synchrotron self-absorption, with the frequency of this turnover varying with the linear scale of the source, although free-free absorption may also play a role (Bicknell et al. 1997). A review of the characteristics of CSS/GPS sources at different wavelengths is given in O'Dea (1998).

The age of CSS/GPS sources has been recently estimated using both the lobe proper motion (e.g. Owsianik & Conway 1998) and the radiative ages (Murgia et al. 1999). The ages derived are less than a few thousand years, therefore strongly supporting the "youth'' scenario in the interpretation of these sources. A summary of the results is given in the review by Fanti et al. (2000).

The sub-galactic size and the young age of these sources makes them an extremely interesting class of radio source. They can be used to probe further the characteristics of the ISM in the early phase of radio source evolution and the effects of the radio plasma expanding in this medium. This has been done in a restricted number of objects both using the atomic hydrogen (e.g. Conway 1996; Pilström 2001; Morganti et al. 2001; Peck & Taylor 2001) and the ionised gas (e.g. Gelderman & Whittle 1994; Morganti et al. 1997; Axon et al. 2000; O'Dea et al. 2002).

An other important aspect in the study of these objects is the comparison of their characteristics with those of the extended radio galaxies. This has been done quite extensively in the radio to investigate the evolutionary scenario. At wavelengths different from radio the available data are still limited. ISO observations (Fanti et al. 2000) and optical and near IR data (de Vries et al. 1998) showed no significant difference between CSS/GPS sources and extended radio galaxies with similar radio power, failing therefore to detect any extra component capable to "frustrate'' the CSS/GPS sources.

To extend the comparison between the optical properties of CSS/GPS and extended radio sources, a group of CSS/GPS sources was selected from a complete sub-sample of the 2 Jy sample of radio sources (see Sect. 2 for details). Optical spectra were obtained for both the compact as well as for the extended sources in this sample (see Tadhunter et al. 1993) and the spectral characteristics could therefore be compared for objects in a similar range of redshift and radio power. A first-order similarity was found between the spectra of CSS/GPS sources and the extended source of similar radio power although the former may be of slightly lower ionisation, possible evidence for the compression effect of shocks due to jet/cloud interaction (Morganti et al. 1997). For the 2 Jy CSS/GPS source sample however, there is a lack of high resolution radio images capable of providing information on their morphology. Most of the well studied CSS/GPS galaxies tend to have symmetric, double-lobed radio structures (e.g. Dallacasa et al. 1995; O'Dea 1998), reminiscent of scale-down versions of extended, edge-brightened Fanaroff-Riley II sources. Collecting this information for the CSS/GPS sources in the 2 Jy sample is particularly important also for the interpretation of their optical data. The presence of more complex radio morphology in these sources (or in some of them) would be an indication of a particularly strong interaction between the radio plasma and the ISM, more than usually found in this type of radio source.

In order to fill this gap, in this paper we present and discuss new high resolution (VLBI and MERLIN) data for this group of seven 2 Jy CSS/GPS sources. These radio images permit a proper morphological classification of the sub-kpc radio emission in these objects.

Figure 1: Radio spectral index plots for all sources.

 

 

Table 1: CSS sources in the sample with a summary of their characteristics.

Name	ID	mV	z	scale	$\log P_{\rm 2.7~GHz}$	$\alpha^{8.4}_{2.7}$
				pc/mas	W Hz-1
0023-263	G	19.5	0.322	2.9	26.91	-0.8
0252-712	G	20.9	0.566	3.7	27.28	-1.25
1151-348	Q	17.8	0.258	2.5	26.56	-0.7
1306-095	G	20.5	0.464	3.5	26.94	-0.85
1814-637	G	18.0	0.063	0.8	25.54	-1.2
1934-638*	G	18.4	0.183	2.0	26.68	-1.15
2135-209	G	19.4	0.635	3.9	27.20	-1.05
* Gigahertz-Peaked Spectrum source (GPS).

Figure 2: Typical uv coverage of the SHEVE array, from the 0252-712 at 2.3 GHz. (No detections to Perth or Hartebeesthoek).