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3. The maps

In the following we present the radio continuum maps of the GRGs. The results for each source are discussed individually. We show two maps per source and frequency, one displaying the total intensity, with vectors proportional to the polarized intensity, the other giving the polarized intensity as contours, with vectors proportional to the percentage polarization. The angle of the vectors is the direction of the electric field. For magnetic field maps we refer to Klein et al. (1994). Detailed studies of spectral indices, rotation, and depolarization measures will be presented in forthcoming papers. We have also compiled a number of point sources observed in the galaxies' fields which probably do not have any physical connection to the GRGs. Their coordinates and flux densities at 609 MHz can be found in Table 3 (click here)-6 (click here).

3.1. NGC 315

The first detailed study of this source was performed by Bridle et al. (1976) using the Arecibo telescope at 0.43 GHz, 1.41 GHz, and 2.38 GHz. Stoffel & Wielebinski (1978) made first measurements with the Effelsberg telescope at 2.7 GHz. Concentrating on the jet, Bridle et al. (1979) observed NGC 315 at 609 MHz, 1.4 GHz, and 4.9 GHz with the WSRT and the VLA. The most extensive study including the polarization data was performed by Willis et al. (1981). A new map at 609 MHz was obtained by Jägers (1987a) using the upgraded WSRT.

   Table 3: Point sources found in the NGC 315 field (at 609 MHz)

The overall structure of NGC 315 suggests precessing beams, although the two lobes are anything but symmetric. The south-eastern lobe is broad, with its northern extension having a steep spectrum: it is not visible at 10.6 GHz. In contrast, the north-western lobe is narrow, with its "back-flowing" extension having a remarkably flat spectrum as suggested by the high-frequency maps, in which it is still easily visible. This peculiar behaviour was already pointed out by Klein et al. (1994), and suggests that a simple precession model is not able to explain the overall morphology and spectral characteristics. While the jet feeding the north-western lobe is bright and permanently visible over the whole frequency range, the counter-jet is only visible intermittently. As will be shown in Sect. 4.1, the spectrum of NGC 315 and its individual components is difficult to determine reliably. In any case, the strong asymmetry of the source, both, in its lobe structure as well as in its jet/counter-jet lengths, strongly suggests different densities/pressures of the intergalactic medium on either side of the host galaxy, rather than an intrinsic cause.

3.2. DA 240

Willis et al. (1974) presented the first high-resolution observations of this source at 609 MHz. Baker et al. (1974) showed a tex2html_wrap_inline3235 map, obtained with the Effelsberg telescope. Spectral index studies have been performed by Strom et al. (1981) who compared maps at tex2html_wrap_inline3237 and 6 cm. Tsien (1982) obtained 0.15 and 1.4-GHz maps of the entire source and high-frequency observations of the famous hot spot. New 609-MHz observations were obtained by Jägers (1987b) with the 3-km WSRT.

Table 4: Point sources found in the DA 240 field (at 609 MHz)

At low frequencies DA 240 is the archetype of a fat double radio galaxy with almost circularly extended lobes, the western lobe having an additional protrusion in the south-western direction. At higher frequencies the lobes shrink to a small straight channel marking the active zone of the source. The hot spot in the eastern lobe, 4C 56.16, is the most salient feature at high frequencies.

3.3. 3C 236

The first high-resolution study was presented by Willis et al. (1974) using WSRT observations at 609 MHz. Strom & Willis (1980) studied the spectral index distribution between 609 MHz, 1.4 GHz, and 4.8 GHz. The large and small scale structure of 3C 236 was the subject of a paper by Barthel et al. (1985) who included both, 1.4-GHz WSRT and VLBI observations for their study. Jägers (1987b) obtained a 609-MHz map with the 3-km WSRT.

At 326 MHz the bridge between the outer lobe areas and the central core is almost closed. The source is very narrow. While most sources of this species reveal low-brightness protrusions in a lateral direction at low frequencies, 3C 236 maintains its straight direction without showing any large-scale curves, wiggles or bends. The observed polarization of the core is an instrumental artifact. The north-western lobe looks more complex in polarized intensity than does the south-eastern one. This is even more evident if one compares the orientations of the electric field vectors.

   Table 5: Point sources found in the 3C 236 field (at 609 MHz)

The overall morphology is characterized by the narrow extent of the lobes perpendicular to the source axis. Comparing the two lobes, however, the north-western one appears broader, a fact which becomes especially evident in the polarized intensity map. The bridge emission between the core and the outer lobes becomes fainter with increasing frequency, which results in a steepening of the spectrum and is indicative of aged particles, a typical behaviour in FRII-radio galaxies.

3.4. 3C 326

The first maps of 3C 326 were presented by Mackay (1969) at 408 and 1407 MHz. Bridle et al. (1972) and Baker (1974) published single-dish observations at 1.4 GHz. The most detailed analysis of this source has been performed by Willis & Strom (1978) who presented maps at frequencies of 609 MHz, 1.4 and 5 GHz.

   Table 6: Point sources found in the NGC 6251 field (at 609 MHz)

Table 7: Integrated flux densities of NGC 315

Table 8: Integrated flux densities of DA 240

The global structure of this source does not vary significantly at different frequencies. The western lobe is narrow, as is particularly evident at 10.6 GHz. The low-frequency WSRT maps do not have sufficient north-south resolution to reveal this, owing to the relatively low declination of 3C 326. The eastern lobe is extended perpendicular to the source axis with a narrow protrusion emerging to the east. The linear polarization maps disclose a more complex structure, with up to three components in the eastern lobe. The point sources at tex2html_wrap_inline3611, tex2html_wrap_inline3613 and tex2html_wrap_inline3615, tex2html_wrap_inline3617 are background sources as already noted by other authors. The eastern component of the western lobe is seen to broaden southwards. The western lobe also shows significant polarization (tex2html_wrap_inline3619) in its central and western components, whereas the broader eastern component of this lobe is more weakly polarized. The large-scale isotropic orientation of the magnetic field is obvious over an extent of tex2html_wrap_inline3621, corresponding to a linear size of 600 kpc.

The overall (projected) source morphology suggests an intrinsically rather symmetric structure, but with the eastern lobe probably bending away from its original orientation, perpendicular to the sky plane.

3.5. NGC 6251

This source was discovered by Waggett et al. (1977) who presented maps of the entire source or only the core at various frequencies between 38 MHz and 15.4 GHz. Stoffel & Wielebinski (1978) published a 2.7-GHz map observed with the Effelsberg telescope. The polarization characteristics have been studied by Willis et al. (1978). Detailed investigations of the jet have been performed by Saunders et al. (1981) and Perley et al. (1984) using high-frequency interferometric maps. Willis et al. (1982) presented a 609-MHz map observed with the WSRT which has been superseeded by new 609-MHz observations with the upgraded instrument (Jägers 1987a).

The north-western lobe appears to consist of two regions, a brighter one which extends symmetrically around the jet, and a fainter part which expands in a south-western direction. This confirms the impression suggested by the 151-MHz map of the north-western lobe (Waggett et al. 1977) that it is very extended in the south-western direction.

The faint part has a steep spectrum, indicating a higher particle age although parts of the very extended structure may have been filtered out in the interferometer maps, especially at 609 MHz. The strong gradients at the northern edges of both lobes delineate the genuine boundaries of the lobes. The jet can be traced out to about 10tex2html_wrap3799. It consists of a luminous part up to 5tex2html_wrap3801 from the core, and a fainter section which curves towards the hot spot. The counter-jet is detected close to the core along the first 3tex2html_wrap3803, disappears and lights up again some 20tex2html_wrap3805 further out. The 326-MHz data confirm this feature being part of the counter-jet on its way to the hot spot as already suggested by Willis et al. (1982). Although less obvious in the 609-MHz map, the connection of feature B to the south-eastern hot spot also becomes clear when this map is smoothed to the resolution of the 326-MHz map. At the lower resolution our 609-MHz map is also consistent with that obtained by Willis et al. (1978).

Table 9: Integrated flux densities of 3C 236

The high-frequency maps emphasize the most active parts of the radio galaxy, viz. the core, the jet, and the hot spots. Unfortunately, the 2.7-GHz and 4.8-GHz maps had to be cut at their north-western and south-western edge (owing to limited observing time) but they can still yield valuable information on intensities and polarization characteristics of the major part of the source. For a complete 2.7-GHz map we refer to Stoffel & Wielebinski (1978), which confirms our results. The polarization maps show that the north-western lobe is strongly polarized, with degrees of polarization as high as 70% (609 MHz). Polarized radiation is detected in most parts of the lobe. In contrast, the south-eastern lobe shows much less extended polarization, but again with high degrees (tex2html_wrap_inline3741). The jet is weakly polarized (tex2html_wrap_inline3743) close to the core. This suddenly changes beyond 33 where the map of polarized intensity shows its maximum. The fractional polarization increases to 23%. It is striking that it coincides with the edge of the diffuse lobe emission. The orientation of the tex2html_wrap_inline3745-vectors indicates the presence of magnetic fields coherent over several arcminutes at 326 MHz. At 609 MHz these scales are even larger, suggesting the patchy structure of a depolarizing sheath.

Table 10: Integrated flux densities of 3C 326

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