The GHz-peaked-spectrum (GPS) radio sources are characterized by a simple convex spectrum which peaks in a range of about a decade around 1 GHz. General discussions on the general properties of these objects are given by O'Dea et al. (1991) O'Dea & Baum (1997), and O'Dea (1998) where an exhaustive bibliography can also be found.
Common characteristics of the bright sample of GPS radio sources are:
small size (
kpc), high radio luminosity, low fractional
polarization, and apparently low variability.
They are a mixed class of quasars and
galaxies. Galaxies tend to be L* or brighter and at redshifts
(O'Dea et al. 1996) while
quasars are often found at very large redshift 
,
(O'Dea 1990).
Currently, there are two main competing hypotheses to explain the origin of GPS radio sources and their possible evolution.
In the "young source" scenario, first suggested by Phillips & Mutel (1982), GPS radio sources with compact double (CD) morphology or compact symmetric morphology (Compact Symmetric Objects: CSO) are classical double radio sources at the very first stage of their lives.
In the "frustration'' scenario, GPS radio sources will never become as large as the classical doubles since they are confined to the sub-kpc scale by a dense and turbulent ambient medium (e.g., O'Dea et al. 1991; Carvalho 1994, 1998). However, it is unclear whether the gas density and its distribution in the nuclear region is sufficient to confine the radio source for times of the order of 107 years (see e.g., O'Dea 1998).
Recent results support the young source hypothesis. Fanti et al. (1995) presented a model for the evolution of the galactic-scale Compact Steep Spectrum (CSS) sources into large scale classical doubles. They argued that the objects showing symmetric morphology are probably not confined by a dense and clumpy medium (see also De Young 1993). They also suggest that the typical CSS source age is of the order of 106 years and that the radio source luminosity decreases by an order of magnitude as the size of the radio source grows from a few kpc to hundreds of kpc (see also Begelman 1996). The same conclusions are reached by Readhead et al. (1996a,b) in their study of a few CSO's. O'Dea & Baum (1997), combining the complete sample of GPS presented here with the CSS sources by Fanti et al. (1990) reach similar conclusions. They further note that the luminosity evolution required makes it likely that GPS and CSS radio sources will evolve into objects less powerful than the most powerful classical doubles.
The detection of arc-second scale faint extended emission around 
10%
of GPS sources (Baum et al. 1990; Stanghellini et al. 1990)
motivated Baum et al. (1990) to suggest that nuclear activity is recurrent
in these sources. In this hypothesis, we see the relic of a previous epoch
of activity as faint diffuse emission surrounding the current young nuclear
source.
In order to achieve a deeper understanding of the GPS radio sources we pursued the following project.
(1) We created a complete sample of GPS radio sources by means of a preliminary bibliographic research, checked with subsequent new multi-frequency data from the Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT).
(2) We determined the properties of the radio spectrum and the polarization.
(3) We obtained optical imaging to determine the host galaxy properties.
(4) We obtained VLBI observations to study the milliarcsecond radio morphology.
In this paper we present the selected sample and the results from the VLA and WSRT radio observations. The optical properties of GPS radio sources are discussed by O'Dea et al. (1990), Stanghellini et al. (1993), and O'Dea et al. (1996). The milliarcsecond morphology is presented in Stanghellini et al. (1997). Constraints on radio source evolution based on our observations of the complete GPS sample are discussed by O'Dea & Baum (1997).
H0 = 100 km s-1 Mpc-1, and q0 = 0.5 have been used in this paper.
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