On the basis of the positional coincidence between the radio nucleus
and Mrk 1498, it is very likely that the identification is correct.
There are two further arguments that the identification is
indeed correct. First, if the host galaxy is not Mrk 1498 but a
background galaxy located more than three times further away
than Mrk 1498, the size of the radio source would be much larger
than the largest radio
source known (3C 236 that is 5.7 Mpc in size, Willis et al
1974). Secondly, the V-magnitude for the galaxy is 14.9 (de Grijp et
al. 1992). Using the relation between redshift and V-magnitude that
Laing (1980) found for 3CR radio galaxies (log z = 0.181V-3.965)
we find an estimated reshift of z = 0.054, consistent with the measured
redshift.
Various models have been proposed to explain the observed differences between radio quasars and radio galaxies. It could be that radio quasars are the progenitors of radio galaxies or that the environment determines whether they becomes a quasar or a radio galaxy (e.g. Norman and Miley 1984). However, most of the observational evidence seem to indicate that the observed differences are mainly due to their orientation relative to the line of sight ("the orientation unification model" e.g. Scheuer 1987; Barthel 1989; Antonucci 1993).
An interesting test for these models is a comparison of the distributions of projected linear radio source sizes for samples of quasars and radio galaxies. Barthel (1989) found that steep spectrum quasars from the 3C catalogue with 0.5 < z < 1.0 have sizes that are systematically smaller than 3C radio galaxies at similar redshift by a factor of 2.2. This difference has been taken as support for the orientation model, indicating a cone angle of 45 deg along which the broad lines of a quasar nucleus can be seem. It is not clear whether this trend also holds at low redshift. It seems that there is a relative shortage of observed steep spectrum quasars at low redshift (Kapahi 1990). This could indicate that the unification scheme breaks down and that there are indeed less quasars than radio galaxies in our local universe. Alternatively, the opening angle of the cone in nearby objects could be less than it is at higher redshifts and therefore less quasars are observed in the nearby universe.
There are two giant radio sources known to have strong broad
permitted lines in their optical spectrum. 0309+411 (z = 0.136) is
1.8 Mpc in size and is identified with a red 18th magnitude
galaxy (de Bruyn 1989). 4C74.26 (z=0.136) is 1.6
Mpc in size
and is identified with a stellar object (Riley et al. 1989). More than
a dozen or so giant radio sources are known (e.g. Saripalli et al.
1986). It seems that the existence of now 3 giant radio sources with
broad lines is broadly consistent with the orientation unification model.
We should stress however that well defined samples of giants are needed to address this issue properly. Since the WENSS survey has excellent sensitivity at low frequency to faint surface brightness limits and covers a large fraction of the northern sky, it will provide large and well defined samples of giants.