4 Discussion and conclusions

Unified Schemes Models consider the low-power FR I radio galaxies as the parent, unbeamed population of BL Lac objects. The boundary between FR I and the powerful FR II radio galaxies has been found to lie around $2\ 10^{25}$ W Hz^-1 at 178 MHz (Fanaroff & Riley 1974). At higher frequencies, like at 1.36 GHz, the segregation in radio power is not as evident as at 178 MHz, and in the literature one commonly finds that $\sim$10^24.5 W Hz^-1 is considered the borderline. Indeed, it is well known (e.g. Bridle 1984, 1987) that this boundary at 1.4 GHz is not really sharp, although one can infer that most of the FR I radio galaxies are below that limit, while most of the FR II lie above. We therefore investigated the properties of the extended emission in the 1 Jy BL Lac sample, and in particular we studied the extended, unbeamed emission.

The monochromatic luminosities derived from our observations are often larger than those found in the literature; furthermore, for a few sources we have revealed extended structure previously unknown. Therefore this work is more complete and goes deeper than previous efforts in this direction.

In Fig. 35 we present a plot of the distribution of the luminosity of the extended, unbeamed emission for the 1 Jy BL Lac sample, as given in Table 3, once the arcsecond scale core has been subtracted out. We have used the measurements derived from our observations, completed with data taken from the literature (see Table 3). On this respect, we chose the better suited images/measurements in order to have about the same accuracy in the subtraction of the arcsecond core emission.

The unbeamed radio luminosities at 1.36 GHz clearly extend beyond the limit (not really sharp at this frequency) of 10^24.5 W Hz^-1separating FR I from FR II radio galaxies, with just a few objects with radio luminosities typical of the brightest FR II galaxies. In general the luminosity distribution of the whole sample seems rather smooth and covers a wide range of radio luminosities.

In a forthcoming paper we will discuss in detail our results, and we will compare the radio properties of the Radio Selected BL Lacs (i.e. the 1 Jy sample) with those of X-ray Selected BL Lacs (i.e. EMSS sample) and further with those of FR I and FR II radio galaxies.