2 Sample selection

2.1 The WENSS survey

The Westerbork Northern Sky Survey is a 325-MHz survey of the sky above $+28\hbox{$^\circ$ }$ declination. About a quarter of this area has also been observed at a frequency of 609 MHz. The unique aspect of WENSS is that it is sensitive to spatial structures over 1 degree on the sky at 325 MHz. The limiting flux density to unresolved sources is about 15 mJy ($5\sigma$) and the FWHM of the beam is $54\hbox{$^{\prime\prime}$ }\times 54\hbox{$^{\prime\prime}$ }{\rm cosec}\,\delta$, with $\delta$ the declination. A detailed description of the observing and data reduction techniques used can be found in Rengelink et al. (1997). The sky area above $+74^{\circ}$ declination has been observed with an increased total bandwidth, so that the limiting flux density to unresolved sources is about 10 mJy ($5\sigma$) in this sky area.

Using the Wieringa (1991) source counts at 325-MHz, the flux density at which the same amount of confusion in the WENSS as Saunders et al. (1987) encountered in the 6C survey can be calculated. For a typical radio source spectral index of -0.8, a similar amount of confusion can be expected in the WENSS at a flux density of ${\sim}400$ mJy, which is almost seven times lower than that for the 6C survey. Similarly, it can be shown that confusion would dominate the selection sources only below 20 mJy in the WENSS survey, which is below its completeness limit of ${\sim} 30$ mJy (Rengelink et al. 1997). This implies that we should be able to efficiently find GRGs in the WENSS down to relatively low flux density levels.

2.2 Selection criteria

In order for a source to be a candidate low redshift GRG, we have used the following criteria. A candidate GRG must have:

1.

an angular size larger than 5 arcminute, and

2.

a distance to the galactic plane of more than 12.5 degree.

The angular size lower limit of $5\hbox {$^\prime $ }$ is the size at which some basic morphlogical information of a source can still be obtained at all declinations the survey has covered. this corresponds to a physical size of ${\sim}750$ kpc at z=0.1, ${\sim}1300$ kpc at z=0.2 and ${\sim}1700$ kpc at z=0.3, and will therefore introduce a redshift-dependent linear size bias in the sample. To avoid high galactic extinction values and confusion by a large surface density of foreground stars, we have restricted ourselves to galactic latitudes above $12.5\hbox{$^\circ$ }$. This results in a survey area of ${\sim}2.458$ steradian ( ${\sim}8100^{\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil ... ...{\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi}$).

Table 1 presents all previously discovered GRGs whose angular size and position on the sky agree with the above selection criteria. The majority of these are smaller than 2 Mpc in size. If their sizes are characteristic for the whole population of GRGs, we thus expect that the majority of selected sources will have a redshift below ${\sim}0.35$. Assuming that the host galaxies are not less luminous than those of the LRL sample of powerful radio sources (Laing et al. 1983) they should be identifiable on the Digitized POSS-I survey (DSS).

2.3 Selection method

Candidate radio sources were selected using a visual inspection of the WENSS radio maps. We preferred this method over possibly more objective, machine controlled selection methods because the complexity of the WENSS radio maps (i.e. the high source surface density and the unavoidable presence of spurious artefacts such as low-level sidelobes of bright sources, etc.) and the wide variety in possible morphologies would make it very difficult to tune such an algorithm. Looking at the maps allows one to easily recognize low-level extended structures in a crowded field.

Above declination $+74\hbox{$^\circ$ }$ we have initially selected our candidates using the earlier available NVSS survey maps (Condon et al. 1998), but we subsequently repeated the selection using the WENSS maps. We found that no WENSS selected candidates were omitted using the NVSS. On the contrary, we have identified two NVSS sources (B1044+745 and B0935+743) that we most likely would not have selected from the WENSS survey alone due to their faintness in the latter. We will elaborate on this when we discuss the selection effects (Sect. 5.1).

2.4 Removing confused sources

The declination dependent beam size results in an unavoidable increase of confusion with decreasing declination. Only with higher angular resolution observations can we determine whether such sources are separate unrelated radio sources. We have used the following additional sources of radio data to achieve this:

First, where available, we have used the 612-MHz WENSS maps which have twice the resolution of the 325-MHz maps. Also the 1.4-GHz NVSS survey, which does not have a declination-dependent beam size and which covers almost the entire area of the WENSS survey, is highly usefull in this respect. Furthermore, we have used the much higher resolution ( $5\hbox{$.\!\!^{\prime\prime}$ }4$ FWHM) maps from the 1.4-GHz FIRST survey (Becker et al. 1995) where available. Also the FIRST survey has mapped a large fraction of the area observed by WENSS, notably the lower declination range away from the galactic plane. Finally, for candidates in areas of the sky where the FIRST survey was not (yet) available, we obtained short 1.4-GHz WSRT observations.

A consequence of the different methods used to eliminate confused sources is that the angular size of the objects are not well determined in all cases. There are two important factors which influence such estimates: First, for edge-brightened sources (FRII-type) high-resolution observations would be required for an accurate measurement, but we do not have these for all such sources and even the ones we have differ in quality and resolution. Second, for FR-I type sources the angular size measured on a map depends strongly on surface-brightness sensitivity. Therefore, sources may have been accidentally removed from the sample because of wrongly estimated sizes.

   
2.5 Identification of the host galaxies

We have used the digitized POSS-I survey (the "Digitized Sky Survey'', DSS) and, in a later stage, also the digitized POSS-II survey to identify the host galaxies of the selected radio sources. The magnitude limit of the red POSS-I plates is ${\sim}20{-}20.5$; the POSS-II is somewhat more sensitive.

Adopting the Cousin R-band magnitude-redshift relation for the host galaxies of the radio sources in the LRL sample (Dingley 1990), we expect to be able to identify host galaxies out to $z\sim 0.5$ (note that the transmission curves of the Cousin R-band and the POSS-E band are much alike).

To identify the host galaxy of a radio source which extends over several arcminutes, a radio core position is often necessary. We have used the WENSS, NVSS, FIRST and our own WSRT radio observations to identify radio cores of the selected sources. For many sources we indeed find a compact central radio source coincident with an optical galaxy in the POSS-I and/or POSS-II.

We cannot rule out that for an individual source the so-found optical galaxy is an unrelated foreground galaxy, and that the actual host galaxy of the radio source is a much farther and fainter galaxy. However, for the sample as a whole, we believe this to be only a minor problem: It would make the radio sources even larger than they already are and the chance of such an occurrence is very small anyway.

For the source B1918+516 the POSS-I plates did not show an obvious host galaxy candidate. Therefore, an optical CCD image has been made by P.N. Best using the LDSS imaging spectrograph on the 4.2-m WHT telescope on La Palma. This image (see Fig. .28) reveals a faint galaxy, close to a relatively bright star. We believe this galaxy to be the host galaxy, due to its proximity to the radio core.

We cross-correlated the positions of the optical galaxies with the NASA Extragalactic Database (NED). In case an optical source with known redshift is found and the resulting size of the radio source is below 1 Mpc, we removed that source from our sample. We present the list of remaining (i.e. after removing sources identified as non-GRGs on basis of NED data) candidate GRG sources in Table. .1. We provide IAU-formatted source names, approximate coordinates of the radio sources, WENSS flux densities, approximate angular sizes and whether we are convinced this a genuine giant radio galaxy candidate on basis of its radio morphology. We remark that many of the WENSS selected sources were rejected after a look at the maps of these sources from the NVSS survey.