1 Introduction

The NRAO VLA Sky Survey (NVSS; Condon et al. 1998) provides radio maps of the sky north of $-40^{\circ}$ declination, at a frequency of 1.4 GHz, in total and polarized intensity, with an angular resolution of 45 $\hbox{$^{\prime\prime}$ }$ and 1 rms = 0.45 mJy/beam. Due to its high sensitivity and resolution compared with other all-sky surveys, the NVSS is a unique tool for the definition of complete samples of extended objects, not known before because of their low flux density and/or because of confusion effects present in previous surveys.

In 1995 we undertook a project for the definition and study of a sample of large angular size radio galaxies selected from the NVSS with the following aims:

i) To construct a sample of radio galaxies with their jets oriented near the plane of the sky and to study the parsec scale properties of these jets. Most samples selected for Very Long Baseline Interferometry (VLBI) observations are usually defined on the basis of a flux density cutoff at high frequencies (e.g. Pearson & Readhead 1988; Polatidis et al. 1995; Taylor et al. 1994; Kellermann et al. 1998; Fomalont et al. 2000), although it is known that the combined effect of such cutoff and the Doppler boosting of the radio emission introduces a bias in those samples towards small orientation angles with respect to the observer's line of sight. As a consequence, only a few sources with symmetric structure on the parsec scale are currently known, despite the importance of these sources to understand the physical properties of parsec scale jets. For the selection of our sample we did not impose any core flux density limitation, and considered that simply due to projection effects, a significant number of large angular size radio sources would be oriented close to the plane of the sky and should show symmetric parsec scale jets.

ii) To study the properties of giant radio galaxies (GRGs; defined as those with a projected linear size $\ge$1 Mpc). This kind of source is difficult to detect: in Fanaroff-Riley type II (FR II; Fanaroff & Riley 1974) giant sources, the association between the two hotspots is often not obvious; on the other hand, the low brightness extended lobes in Fanaroff-Riley type I (FR I) sources can only be detected with deep radio observations and therefore, such sources are missed in most radio surveys. Ishwara-Chandra & Saikia (1999) collected a sample of 53 GRGs from inhomogeneous literature data, but only recently, the NVSS and the Westerbork Northern Sky Survey at 325 MHz (WENSS; Rengelink et al. 1997) provide sufficient sensitivity to detect GRGs in a systematic way. Schoenmakers (1999) presented a sample of 47 GRGs from the WENSS survey. A new sample of large angular size radio galaxies will contain an appreciable number of intrinsically large radio galaxies and will allow us to increase the number of known GRGs. A large sample with homogeneous selection criteria is necessary to study the poorly known properties of GRGs, and how their jets interact with the external medium at large distances from the parent galaxy (Ishwara-Chandra & Saikia 1999; Schoenmakers et al. 2000; Lara et al. 2000).

iii) To investigate the evolution of radio galaxies. The selection of a sample of large angular size radio galaxies and the comparison with other samples will give new information on the time evolution of radio sources, since a larger source size implies, in principle, a higher probability of selecting older sources than in other samples (Ishwara-Chandra & Saikia 1999; Schoenmakers et al. 2000).

We present here (Paper I) the sample definition and members, VLA radio maps of 79 selected objects and the main physical parameters of the sources, and we discuss the effects of the selection criteria on the sample. Papers II and III (in preparation) will present optical data (images and spectroscopy) of the associated galaxies and an analysis of the sample properties with statistical considerations, respectively.