4 First results

4.1 The number of GRGs

Of the 33 candidate sources which we have been able to identify spectroscopically, only three, and possibly five have projected linear sizes below 1 Mpc. These are B0217+367, B1709+465 and B1911+479. The uncertain cases are the source B0905+352, for which we do not have a well determined redshift yet and B1736+375 which may consist of two unrelated radio sources. Together with the 19 known GRGs in the area of the WENSS which share our selection criteria, we have so far identified 47 GRGs. This is by far the largest sample of sources with a projected linear size above 1 Mpc selected from a single survey. We also mention the existence of the large, but highly incomplete sample of GRGs which has been compiled by Ishwara-Chandra & Saikia (1999) from the literature. Although of similar size, the sample presented in this paper is better suited for statistical investigations of the radio and optical properties of GRGs since it has been selected from a single survey and in a more uniform matter.

4.2 Distribution functions of the sample

4.2.1 Flux density distribution

Figure 1a presents a histogram of the 325-MHz flux density distribution of the sample of 47 sources. The hatched bars indicate the flux density distribution of the sample of previously known GRGs. Not surprisingly, we find that all newly discovered GRGs have flux densities below 3 Jy, only; all ten GRGs with a 325-MHz flux density above this value had already been identified as such. For the area of the sky covered by WENSS, we can therefore agree with Riley (1989) who argued that no bright extended sources are missing from the LRL sample of bright radio sources ( S₁₇₈>10.9 Jy, Dec $>+10\hbox{$^\circ$ }$, $\vert b\vert>10\hbox{$^\circ$ }$). Furthermore, we have extended the range of flux densities at which low redshift GRGs are found to sub-Jy levels. The median flux density of the combined sample is 1.15 Jy, which is almost a factor of four below the median value of the sample of previously known GRGs (see Table 2).

Figure 1: Histograms of several properties of the new and old sample of GRGs. In all plots, the hatched bars indicate the distribution of previously known GRGs (see Table 1). a) (upper left): The 325-MHz flux density distribution of the GRGs. We have used a binsize of 0.2 in the logarithm of the flux density in Jy. b) (upper right): The redshift distribution of the GRGs, using a binsize of 0.03 in redshift. The source 8C0821+695 at z=0.538 lies outside the range of this plot. c) (lower left): The projected linear size distribution of the GRGs, using a binsize of 0.2 Mpc. The source 3C236 (D=5.7 Mpc) lies outside the range of this plot. d) (lower right): The rest-frame 325-MHz radio power distribution of the GRGs, using bins of width 0.2 in the logarithm of the radio power in WHz-1.

4.2.2 Redshift distribution

The new sample of GRGs mostly contains GRGs at (see Fig. 1b). The only exceptions are the sources 8C0821+695 (z=0.538; Lacy et al. 1993), B0750+434 (z=0.347) and B0925+420 (z=0.365). The redshift distribution peaks at $z \sim 0.1$. The decrease in the number of sources towards higher redshift is likely to be due to the lower angular size limit used in the selection of the sample. Further, only more powerful GRGs will be selected towards higher redshift, of which the space density is likely to be lower. The median redshift of the sample of new GRGs is 0.1404, which is higher than that of the old GRGs alone (0.099; see Table 2). Since the average flux density is lower this is not surprising. The median redshift of the combined sample is 0.1175.

4.2.3 Linear size distribution

No source with a (projected) linear size exceeding that of the GRG 3C236 has been found. This source is therefore still the largest known radio galaxy in the Universe. In Fig. 1c we have plotted the (projected) linear size distribution of our sample. The median values of the linear sizes of the old and new GRGs lie close together (see Table 2). The majority of sources have linear sizes between 1 and 2 Mpc; the distribution of the combined sample falls off strongly at a linear size of 2 Mpc. This sharp decrease was not clear from the sample of "old'' GRGs only, due to the small number of sources. In Sect. 5.4 we will discuss whether the observed cut-off is a result of selection effects or an intrinsic property of the population of GRGs.

4.2.4 Radio power distribution

We calculated the emitted radio power at a rest-frame frequency of 325 MHz assuming isotropic emission. Since the redshifts are low, the radio K-correction is only small. For the new GRGs the measured spectral index between 325 and 1400 MHz has been used; for the "old'' GRGs a spectral index of -0.8 has been assumed if no reliable literature value could be found. The distribution of 325-MHz radio powers has been plotted in Fig. 1d. Despite the fact that a large number of GRGs has been found at flux density values well below that of the "old'' sample, the distribution of the radio powers of the new sources largely overlaps that of the old sources. This is related to the, on average, higher redshifts of the new sources. The combined sample is distributed rather uniformly between 10²⁵and 10²⁷ WHz^-1; the sharp peak at the radio power bin centered on 10^26.1 WHz^-1 is most likely a result of small number statistics. At 325 MHz, the traditional separation between FRII and FRI-type sources (Fanaroff & Riley 1974) lies near 10²⁶ WHz^-1, which is close to the median value of the combined sample (see Table 2).

  

Table 2: Median values of some of the properties of the GRGs in the sample of "old'' GRGs, the sample of new GRGs and the combined sample of 47 sources. Column 1 gives the property; Cols. 2 to 4 give the median value of these properties for the old, new and combined sample, respectively.

$$\begin{tabular}{l l l l} \hline\hline% \multicolumn{1}{c}{(1)... ... Redshift & 0.099 & 0.1404 & 0.1175 \\ \hline % \end{tabular}$$