Properties of the giant

The radio source discussed in this paper has been discovered in the WENSS survey maps at 325 MHz. The data for the map that contain the large radio source has been taken during the winter of 1992. A contour map of the radio source is presented in Fig. 1. The resolution of the map is 54 x 68 arcsec and the rms noise is 3.3 mJy beam. The radio source has a classical double-lobed structure, with an unresolved compact source in the middle. The distance between the location of the maxima of emission within the two lobes is 18.4 arcminutes. The total flux density of the source is = 1.9 Jy. The compact source at the centre is unresolved and has a peak flux density of 25 mJy beam.

We have estimated the total flux density at 4850 MHz using the NRAO Green Bank survey (Condon et al. 1989). A gaussian function was fitted to the northern and southern lobe and to the compact source between the two lobes. Summing the flux of these three components, we find that the total flux at 4850 MHz for the source is = 320 mJy, yielding a spectral index =-0.66. The peak flux density of the compact component is = 27 mJy beam, giving a spectral index of = 0.0. We therefore assume that this component is the flat spectrum core. Its coordinates as measured from the WENSS map are (B1950)

The object was also seen in the 6C survey (151 MHz) as two separate sources (Hales et al. 1990). The lack of spatial resolution and sufficient sensitivity prevented its recognition as a single object. The total flux density of the source at 151 MHz is 3.13 Jy, consistent with the radio spectrum being a simple powerlaw from 151 MHz down to 4850 MHz with a spectral index = -0.66.

The optical position of the Markarian Galaxy 1498 is (B1950) (Kojoian et al. 1984). This is well within the errors of the position of the core of the radio source, strongly suggesting that this radio source is associated with this Markarian galaxy. In Fig. 2 we show in grey scale a part of the Palomar O survey around Mrk 1498 as digitized by the APM plate measuring machine (Irwin et al. 1994). The contours on this figure are the central part of the WENSS image of the radio galaxy (e.g. Fig 1). The central object is Mrk 1498. The extent of Mrk 1498 on the POSS is at least 40 arcsec, with no spiral structure visible indicating that Mrk 1498 is an elliptical galaxy (see also Low et al. 1988). It is associated with IRAS1612+518 at redshift of 0.0547 (e.g. de Grijp et al. 1992). If the identification is correct, the projected size of the radio source is 1.6 Mpc. Its bolometric infrared luminosity L(IR) = (Low et al. 1988) is typical for a nearby radio galaxy (Knapp et al. 1990).

We have taken an optical spectrum of Mrk 1498 on June, 28, 1995 Using the ISIS spectrograph on the 4.2-m William Herschel Telescope (WHT). The object was observed through a slit that had a width of 2 arcsec. The observation was made with the 540 nm dichroic in place; On the blue as well as the red arm we used a Tek chip. The conditions were photometric. The analysis was carried out in a standard way using the iraf twodspec package. In our analysis of the spectrum we have followed Rottgering et al. (1995). The resulting spectrum has a pixel size of 5.7 Å, a wavelength range from 3374 - 8266 Å, a resolution of 12 Å.

and is shown in Fig. 3. The spectrum resembles that of a narrow emission line radio galaxy (e.g. Osterbrock 1989); the identification of stronger emission lines are indicated in Fig. 3. From the stronger forbidden lines we find that the redshift of Mrk 1498 is z = 0.056 + 0.001, roughly consistent with the measurements of de Grijp et al. 1992. The (deconvolved) full width at half maximum of all the lines is typically 400 km s, typical for a narrow line radio galaxy. The part of the spectrum that contains H is shown in Fig 4: H is the only permitted line that has a faint broad component (full width at zero intensity of 6500 km.

On the basis of data with less signal to noise and/or less resolution, the spectrum of this object has been classified as a Seyfert 1 (Markarian et al. 1983; Low et al. 1988; de Grijp et al. 1992). Given our deep spectrum, the classification of Mrk 1498 as a Seyfert 2 is more accurate. Since Mrk 1498 is associated with a powerful radio source, we classify this object as a narrow line radio galaxy.