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Subsections

4 Discussion and conclusions

We have presented in this paper new, higher resolution radio images of a group of 14 galaxies belonging to the 2-Jy sample of radio sources. The new images improve on the data already available for these objects and, in general, the database that we are building up on the sample.

Although the radio data presented here are mainly useful in the context of the work we are doing on the sample, it is still interesting to highlight some general results that can be derived from the new data. These will be briefly described in the following sections.

4.1 The radio cores

From our previous studies we have found that the core dominance [i.e. ratio between the core and the extended radio fluxes $R=S_{{\rm core}}/(S_{{\rm tot}}-S_{{\rm core}})$, in tests of unified schemes commonly used as an indicator for the orientation of a source] appears to depend on both the morphological classification (Morganti et al. 1995) and the optical characteristics of the radio galaxies. Among the FRII radio galaxies, most Narrow Line Radio Galaxies (NLRGs) show low values of $\log R$, while BLRGs tend to have large $\log R$ (Laing et al. 1994; Morganti et al. 1997a; Hardcastle et al. 1998) supporting the idea that BLRGs are more beamed toward us.

What do the new radio data tell us? In the observed sample we have three BLRG: 0035-02, 1602+01, 1938-15 (two of them only recently classified as BLRG). Both 0035-02 and 1602+01 have very prominent cores and the core flux densities measured in the new observations are consistent with what was found before. In particular we find R=0.33 and R=0.061 for 0035-02 and 1602+01 respectively (if using the total fluxes taken from single dish observations, Peacock & Wall 1985). On the other hand, in 1938-15 we do not detect any core in our higher resolution map. This sets an upper limit to the value of R (core flux $\raisebox{-0.6ex}{$\,\stackrel
{\raisebox{-.2ex}{$\textstyle <$ }}{\sim}\,$ }$ 1.5 mJy; $R~\sim$ 0.0004), well below the average value for BLRG ( $\sim 0.027$). Thus, this object appears to be an exception like 0347+05 ( $R_{2.3\, {\rm GHz}} < 0.0006$) as pointed out by Tadhunter et al. (1998). These objects deserve some follow-up work to understand their real nature and why they show such differences from the other BLRG.


  \begin{figure}\begin{tabular}{cc}
{\psfig{figure=ds1732f16a.eps,angle=0,width=5cm}\psfig{figure=ds1732f16b.eps,angle=-90,width=5cm} }
\end{tabular}\end{figure} Figure 16: Zoom-in of the jet (Right) and of the southern lobe (Left) of 0034-01 at 6-cm. with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 1.0 ratio). The contour levels are: $0.25 \times -1$, 1, 2, 4, 8, 16, 32, 64, 128 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f17.eps,angle=0,width=8.8cm} }
\end{figure} Figure 17: Image of 0035-02 at 6-cm. The contour levels are: $0.6 \times -1$, 1, 2, 3, 4, 6, 8, 10, 15, 20, 25, 30, 50, 100, 150, 200, 400, 600, 800 mJy beam-1. The peak flux is 514.6 mJy beam-1. The cross indicates the position of the optical galaxy

4.2 Asymmetry in depolarization

For the objects for which we have two frequencies available, i.e. the objects observed with ATCA, we could investigate the presence of asymmetries in the depolarization. This asymmetry has been found in 0039-44, 0409-75 and 1938-15 and it is also notable that these are among the highest redshift sources in the sample and they are all small (i.e. $\raisebox{-0.6ex}{$\,\stackrel
{\raisebox{-.2ex}{$\textstyle <$ }}{\sim}\,$ }70$kpc) radio sources. No obvious correlation has been found between the depolarization and the RM(but see later for 0409-75).

If the depolarisation is produced by an external Faraday screen (as generally believed), this screen can be due to either the X-ray halo around the radio source or to gas associated with the radio galaxy itself. In the latter case, we do not have the resolution high enough (specially in the ATCA data) to investigate in detail the structure of DP and RM in the sources.

On the other hand, if the depolarisation is due to the X-ray halo around the radio source we can estimate the Faraday dispersion ($\Delta$) for our small sources from the formula given in Garrington et al. (1991): $DP = {\rm exp}[-2k^2\Delta^2(\lambda_1^2-\lambda_2^4)/(1+z)^4]$ where DP is the depolarization and k=0.81 if we want to obtain $\Delta$ in units of cm-3 $\mu$G pc. For our frequencies we derive $\Delta = 270 (1+z)^2(-\ln DP)^{1/2}$ and we find values of the Faraday dispersion ranging from $\sim 800$ cm-3 $\mu$G pc for the E lobe of 0409-75 to $\sim 240$ cm-3 $\mu$G pc for the E lobe of 0039-44. These values are typically higher than what found for extended sources (Garrington et al. 1991; Morganti et al. 1997b) but they lie on the established trend found by Garrington & Conway (1991) between the linear size and the Faraday dispersion as do the CSS (Garrington & Akujor 1996). For the CSS, this result is expected if they represent simply young version of the extended radio galaxies but living in a similar kind of environment. In this case, the trend between size and $\Delta$ would represent the radial decline of the density of the environment and the small radio sources in our sample could be, to first order, seen as an evolved phase of CSS on their way to becoming extended objects.


Of the three objects considered above, the only case in which both a large depolarization and a large RM has been observed is 0409-75. It is interesting to note that this object also shows a quite low ionization state (e.g. [O III] $\lambda$5007/[O II] $\lambda$3727 = 0.19, Dickson 1997) for a galaxy of such a high radio power. Thus, in the case of this object the weakness of [O III] $\lambda$5007 could be due to a low ionization parameter, for instance, due to an interaction between the radio jet and a particularly rich ambient gas.


  \begin{figure}\begin{tabular}{cc}
{\psfig{figure=ds1732f18a.eps,angle=-90,width=6cm}\psfig{figure=ds1732f18b.eps,angle=-90,width=6cm} }
\end{tabular}\end{figure} Figure 18: Image of 0035-02 at 6-cm with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 1.0 ratio). The contour levels are: $0.6 \times -1$, 1, 2, 4, 8, 16, 32, 64, 128 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f19.eps,angle=-90,width=9cm} }
\end{figure} Figure 19: Image of 0117-15 at 6-cm with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 0.081 ratio). The contour levels are: $0.5 \times -1$, 1, 2, 4, 8, 16, 32, 64, 128 mJy beam-1. The peak flux is 97.0 mJy beam-1. The cross indicates the position (from di Serego et al. 1994) of the optical galaxy


  \begin{figure}{\psfig{figure=ds1732f20.eps,angle=0,width=9cm} }
\end{figure} Figure 20: Image of 0442-28 at 6-cm. The contour levels are: $0.45 \times -1$, 1, 2, 3, 4, 6, 8, 10, 15, 20, 25, 30, 50, 100, 200, 400 mJy beam-1. The peak flux is 197.0 mJy beam-1


  \begin{figure}\begin{tabular}{cc}
{\psfig{figure=ds1732f21a.eps,angle=-90,width=7cm}\psfig{figure=ds1732f21b.eps,angle=-90,width=6cm} }
\end{tabular}\end{figure} Figure 21: Zoom-in of the southern (Right) and of the northern lobe (Left) of 0442-28 at 6-cm. with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 1.0 ratio). The contour levels are: $0.45 \times -1$, 1, 2, 4, 8, 10, 15, 20, 25, 30, 40, 50 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f22.eps,angle=-90,width=9cm} }
\end{figure} Figure 22: Image of 0453-20 at 6-cm. The contour levels are: $0.40 \times -1$, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50,75 mJy beam-1. The peak flux is 30.4 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f23.eps,angle=-90,width=9cm} }
\end{figure} Figure 23: Image of 0453-20 at 6-cm with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 1.7 ratio). The contour levels are: $0.5 \times -1$, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20, 25, 30, 40, 50, 75, 100 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f24.eps,angle=-90,width=9cm} }
\end{figure} Figure 24: Image of 1602+01 at 6-cm. The contour levels are: $0.45 \times -1$, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20, 25, 30, 40, 50, 75, 100, 125 mJy beam-1. The peak flux is 94.5 mJy beam-1


  \begin{figure}\begin{tabular}{cc}
{\psfig{figure=ds1732f25a.eps,angle=-90,width=9cm}\psfig{figure=ds1732f25b.eps,angle=-90,width=9cm} }
\end{tabular}\end{figure} Figure 25: Image of 1602+01 at 6-cm with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 0.5 ratio). The contour levels are: $0.45 \times -1$, 1, 2, 4, 8, 16, 32, 64 mJy beam-1. The peak flux is 94.5 mJy beam-1


  \begin{figure}{\psfig{figure=ds1732f26.eps,angle=-90,width=9cm} }
\end{figure} Figure 26: Image of 2314+03 at 6-cm. The contour levels are: $0.45 \times -1$, 1, 2, 4, 8, 16, 32, 64, 128, 256 mJy beam-1. The peak flux is 400.7 mJy beam-1


  \begin{figure}\begin{tabular}{cc}
{\psfig{figure=ds1732f27a.eps,angle=-90,width=...
...}\psfig{figure=ds1732f27b.eps,angle=-90,width=6.5cm} }
\end{tabular}\end{figure} Figure 27: Zoom-in of the eastern (Left) and of the western lobe (Right) of 2314+03 at 6-cm with superimposed vectors indicating the projected electric field direction. The vectors are proportional in length to the fractional polarisation (1 arcsec = 0.25 ratio for the eastern lobe and 1 arcsec = 0.5 ratio for the western lobe). The contour levels are: $0.45 \times -1$, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512 mJy beam-1


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