WNB 0313+683 : analysis of a newly discovered giant radio galaxy

We present the results of a detailed analysis of the newly discovered 15' large radio galaxy WNB 0313+683. It has been discovered in the WENSS and NVSS radio surveys, and has been identified with an optical galaxy at a redshift of 0.0901 +/- 0.0002. The linear size of the radio structure is 2.0 Mpc, which places it in the class of Giant Radio Galaxies (GRGs). Radio observations have been carried out with the WSRT at 1.4 GHz and with the 92-cm broadband system, with the 100-m Effelsberg telescope at 10.45 GHz and with the VLA at 1.4 GHz and 5 GHz. At 10.45 GHz the core is the dominant source structure, emitting similar to 25% Of the total flux. It has an inverted spectrum with a spectral index of alpha = +0.42 +/- 0.03 (S-nu proportional to nu(alpha)) between 1.4 GHz and 10.45 GHz, suggesting a very compact structure, although we can not rule out variability of the core luminosity. The Rotation Measure distribution has been mapped using a new method and has been found to be very uniformly distributed over the source. It is therefore probably galactic in origin, with only a small contribution from the (surroundings of the:) source itself. From the spectral index distribution we have de;ermined an upper limit on the spectral age of 1.4 +/- 0.1 x 10(8) yrs. The particle density of the ambient medium, using ram-pressure equilibrium at the hotspots, is greater than or similar to 1.6 x 10(-6) cm(-3) for the southern lobe and greater than or similar to 5.8 x 10(-7) cm(-3) for the northern lobe. An independent measure of the external density has been determined using the amount of depolarization towards the southern lobe. This gives a density, averaged along the line of sight, which is a factor of 10 higher (2.0 x 10(-5) cm(-3)) than the density near the head of the lobes found using the ram-pressure arguments. This discrepancy might at least partly be the result of a contribution from internal depolarization, which we can not exclude on basis of our radio data. From spectroscopical observations of the host galaxy we find that the Hoc emission line has a broad component, and that the extinction must be large with colour index E(B - V) = 0.98 +/- 0.10 mag. Since the galactic latitude is +9.degrees 8, the extinction is probably mostly galactic in origin. We further find that WNB 0313+683 has a very large optical emission line flux with respect to its estimated jet power, when compared with the correlation between these two properties found by Rawlings Br Saunders (1991). We argue that this, together with the relatively high radio power and the inverted radio spectrum of the radio core, is suggestive of a new phase of radio activity in WNB 0313+683.