Astron. Astrophys. 336, 455-478 (1998)

7. Summary and conclusions

We have investigated the newly discovered large radio source WNB 0313+683. This  source was discovered in the WENSS. The radio core has been identified with a weak optical galaxy. It has a redshift of , and thus a projected linear size of 2.0 Mpc. The optical spectrum is typical of a narrow-line radio galaxy. The H-line has a broad ( km s^-1) component. The color index, determined from the (narrow) H/H line-ratio, is large:  mag, where we have assumed that the extinction is mostly galactic (the galactic latitude is +9.8^o). The HI column density towards the host galaxy, measured in the Leiden-Dwingeloo galactic HI survey, translates into a color index of 0.51 mag. But the large variations observed on scales of  in the HI survey maps suggest that denser regions may well exist within a single beam.

In addition to the WENSS and the NVSS 1.4-GHz maps, we have observed WNB 0313+683 with the 100-m Effelsberg telescope at 10.4 GHz, with the WSRT using the 92-cm 8-channel broadband system, and with the VLA at 1.4 and 5 GHz. The VLA and Effelsberg observations show that WNB 0313+683 is a core-dominated radio galaxy. The radio spectral index of the core is , at least up to 10.4 GHz, at which frequency it contributes to the total flux of the source. The prominence of the core, if due to relativistic beaming, suggests that the radio axis of the (sub)arcsec core structure is oriented at an angle to the plane of the sky. If the radio axis of the source as a whole is similar to the radio axis of the core, the deprojected linear size would be 2.6 Mpc.

Spectral index profiles have been made along the radio axis. They have been used to estimate the age and advance velocities of the two lobes. We find that the source has a maximum age of yrs. The found advance velocities of the lobes are for the northern lobe, and for the southern lobe. The ambient densities have been estimated using ram-pressure equilibrium at the heads of the jet, and by assuming that the impact area is limited to the observed size of the hotspots. We find lower limits to the particle density of  cm^-3 for the material surrounding the southern hotspot and  cm^-3 around the northern hotspot. The contrast in the external density of the two lobes is reflected in the lobe luminosities. The southern lobe is much brighter (by a factor of ) and appears more confined than the northern one.

We have studied the Rotation Measure and depolarization towards WNB 0313+683. The Rotation Measures have been determined by a new technique, called Rotation Measure Mapping. We find a very smooth RM distribution over the southern lobe, with a mean value of  rad m^-2. We believe that this is of galactic origin. After subtracting this mean value from the observed values, we only find significant residual RM values towards the central bulge ( rad m^-2).

Depolarization towards WNB 0313+683 is only important at wavelengths above 21cm, since we measure almost no depolarization between the Effelsberg and the NVSS data but find significant depolarization between the NVSS and the 92-cm WSRT data. We have discussed the origin of the depolarization and conclude that it could either be caused inside the radio source itself, or by a large magneto-ionic cluster-like halo surrounding the source. The current data do not allow us to distinguish between these two scenarios. We have investigated the distribution of the depolarization, and the derived Faraday depth , towards the southern lobe. We find no strong increase of near the core, as would be expected if the host galaxy lies at the center of a cluster halo with a dense core, but the values are significantly higher than those near the southern hotspot. Overall, is close to 1.0 cmG pc.

In the case that a large halo surrounding the radio source is responsible for the depolarization, we estimate a lower limit for the electron density, averaged along the line of sight towards the southern lobe, of  cm^-3. This is an order of magnitude above the values we find in the spectral index analysis. Part of this might be caused by a contribution from internal depolarization. Our data do not exclude this possibility.

In many respects, WNB 0313+683 is like any other GRG. Its size is close to the median value of the `classical' GRGs ( Mpc), the density of the ambient medium is low, of the order of  cm^-3, the spectral age is large ( yrs), there is only little depolarization, and the Rotation Measures, corrected for the galactic contribution, are low as well ( rad m^-2).

The inverted spectrum radio core and the very strong optical emission lines as compared to the estimated jet power, suggest an interrupted radio activity of the AGN. VLBI observations are necessary to investigate the structure of the radio core in more detail.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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