1 Introduction

The central activity in a fraction of Active Galactic Nuclei (AGN) is capable of producing relativistic outflows of matter, the so-called "jets'', for a prolonged period of time, possibly up to a few 10⁸ yr. These jets, when powerful enough, inflate a cocoon (e.g. Scheuer 1974; Falle 1991) which expands first in the Interstellar Medium (ISM) and later in the Intergalactic Medium (IGM) of the host galaxy. The evolution of such cocoons is traced by the radio lobes, which in themselves are only a (albeit important) "side effect'' caused by the presence of magnetic fields in the cocoon.

The giant radio galaxies (GRGs) are those radio sources whose lobes span a (projected) distance of above 1 Mpc. As such, GRGs must represent a late phase in the evolution of radio sources. Models of radio source evolution (e.g. Kaiser et al. 1997; Blundell et al. 1999) predict the radio power and linear size evolution of powerful radio sources with time. According to these models, GRGs must be extremely old (i.e. typically older than 10⁸ yr) and probably also located in underdense environments, as compared to smaller radio sources of comparable radio power (e.g. Kaiser & Alexander 1999).

Multi-frequency radio observations (Mack et al. 1998) have shown that spectral ages of GRGs are of the same order as expected from source evolution models. It is, however, not clear at all whether spectral ages are representative of the dynamical ages (e.g. Parma et al. 1999). This questions the validity of radio-based determinations of the properties of the environments of these sources. Still, constraints on the environments of GRGs are of high importance since the radio lobes of these sources penetrate deeply into the intergalactic medium. It is almost impossible to find the properties of this medium, otherwise than from studies of such radio lobes (e.g. Subrahmanyan & Saripalli 1993).

A major problem for such studies is that currently known GRGs have not been uniformly selected. The difficulties encountered while selecting extended radio sources have been demonstrated by Saunders et al. (1987), who have searched for GRGs in a small region of the 151-MHz 6C survey. The 6C survey, with only 30 mJy beam^-1 RMS-noise and a beamsize of $4.2\hbox{$^\prime$ }\times4.2\hbox{$^\prime$ }\,{\rm cosec}\,\delta$ FWHM (with $\delta$ the declination) has an excellent sensitivity to large, faint objects. However, using higher resolution observations they found that only at integrated flux densities above 5 Jy has a radio source larger than $5\hbox {$^\prime $ }$ a good chance of being a genuine GRG in the 6C. At a flux density level of 1 Jy, Saunders et al. find that most of the sources which appear as large extended structures on the 6C survey maps are the result of confusion of physically unrelated sources. Their work demonstrates that 1) an efficient search for GRGs has to be done with sufficient angular resolution to minimize confusion problems and that 2) it should be done with a high sensitivity, also for large-scale structures (up to a few tens of arcmin) on the sky. The recently completed WENSS survey (Rengelink et al. 1997) meets both these demands.

In this paper we report of the selection of new giant radio sources from the WENSS. Subsequent papers will present additional radio observations (Schoenmakers et al. 2000b), a more detailed analysis of the spectroscopic data and a discussion of the evolution of GRGs (in preparation, but see Schoenmakers 1999a). In Sect. 2 we outline the selection technique and criteria. Section 2.5 presents the strategy we have adopted for finding the optical identifications, and Sect. 3 describes the spectroscopic observations of these identifications. In Sect. 4 we present the first results of the new sample of GRGs: Flux densities, linear sizes, redshifts, etc. A discussion of these results and on the sensitivity of the WENSS survey to extended radio sources is given in Sect. 5.

Throughout this paper, a spectral index $\alpha$ is defined according to the relation $S_{\nu} \propto \nu^{\alpha}$ between flux density $S_{\nu}$ at frequency $\nu$, and the frequency $\nu$.

  

Table 1: List of all previously known GRGs (i.e. before 1998) in the area of the sky covered by the WENSS and fulfilling our selection criteria. Column 1 gives the name of the source in IAU-format; Col. 2 gives the more common name of the source, if available; Cols. 3 and 4 give the coordinates of the host galaxy in B1950.0; Cols. 5 and 6 give the redshift of the host galaxy and a reference to it; Col. 7 gives the projected linear size of the radio source in Mpc; Col. 8 designate the morphological type of the radio source, i.e. FRI or FRII or an intermediate type, FRI/II. If a "B'' is added, it means that the host galaxy is a broad-line object, a "Q'' means that it is a quasar. Column 9 gives a reference to radio maps in which the radio morphology can be studied. Columns 10 and 11 give the flux density at 325 MHz and the reference for this value.

$$\begin{tabular}{l l l l l l c l l r@{$\,\pm\,$}r l} \hline\h... ... & R88 & 4.76 $\pm$ 0.10 & WE \\ \hline \hline % \end{tabular}$$

References: B72: Burbidge & Strittmattar (1972); B81: van Breugel & Willis (1981); B82: van Breugel & Jägers (1982); B89: de Bruyn (1989); C75: Colla et al. (1975); D70: Demoulin (1970); D90: Djorgovski et al. (1990); DJ95: Djorgovski et al. (1995); FB78: Fomalont & Bridle (1978); G92: de Grijp et al. (1992); H79: Hine (1979); J86: Jägers (1986); M79: Miley & Osterbrock (1979); MA79: Masson (1979); M96: Marcha et al. (1996); M97: Mack et al. (1997); P84: Perley et al. (1984); P96: Parma et al. (1996); R88: Riley et al. (1988); R96: Röttgering et al. (1996); S73: Sargent (1973); S80: Strom & Willis (1980); S81: Strom et al. (1981); S82: Saunders (1982); S83: Strom et al. (1983); S85: Spinrad et al. (1985); S86: Saripalli et al. (1986); S87: Saunders et al. (1987); S97: Simien & Prugniel (1997); V89: Vigotti et al. (1989); WE : WENSS (measured in the radio map); W74: Willis et al. (1974); W77: Wagett et al. (1977); W81: Willis et al. (1981).