Giant radio galaxies (GRGs) form an extreme class of extragalactic radio sources. They can be studied in detail because of their large angular size. Their huge intrinsic sizes must be due either to very powerful AGNs or to a surrounding medium of very low density. These large sizes imply long evolution times which can be investigated by multi-frequency observations. This type of source also allows us to study characteristics of the intergalactic medium as the source expands into the host galaxy's surrounding environment.
Numerous studies of GRGs have been performed especially in the
low-frequency regime (e.g. Willis & Strom 1978;
Willis et al.
1978; 1981; Bridle et al. 1979; Strom & Willis
1980; Barthel et al. 1985; Jägers 1986; 1987a,b).
At higher frequencies there have been measurements with the Effelsberg 100-m
telescope at 
and 6 cm by Baker et al.
(1974), Stoffel & Wielebinski (1978),
Strom et al.
(1981), Klein et
al. (1994), and Saripalli et al. (1996).
Since particle aging first affects higher electron energies, low-frequency observations mainly show the unevolved stage of the sources. Knowledge of the low-frequency spectral indices is essential to fit spectral aging models, which will be reported in forthcoming papers.
High-frequency observations as reported by Klein et al. (1994) and Saripalli et al. (1996) are essentially free of Faraday effects and thus enable us to directly "map'' the intrinsic (projected) magnetic field. A comparison of these observations with the low-frequency measurements allows us to determine the degree of disorder and (de-)polarization characteristics. The very high degree of polarization which is a general characteristic of the sources chosen in this study provide significant polarization information even at 326 MHz.
Here we report observations of the GRGs NGC 315, DA 240, 3C 236, 3C 326,
and NGC 6251 at 
and 49 cm, carried out with
the Westerbork Synthesis Radio Telescope (WSRT) and 
, 6.3 cm, and 2.8 cm using the Effelsberg 100-m telescope. The WSRT
observations and results had previously been presented in brief by Willis
& O'Dea (1990). The 10.6-GHz data had been published by
Klein et al. (1994), but the maps had not yet been cleaned
with the antenna pattern, which has been performed in the meantime.
In Sect. 2 we describe the observations and the data analysis. In Sect. 3 the
maps of total intensity and linear polarization are presented, along with a
brief description of their most striking characteristics. In Sect. 4 we have
compiled the integrated flux densities of the sources and source
components to derive their integrated spectra.
More detailed studies will be presented in forthcoming papers.
Throughout this paper we use 
and
.
Table 1: Map parameters of the WSRT maps
Table 2: Map parameters of the Effelsberg maps