The spectral indices in Cen A are typical for nonthermal emission
produced by the synchrotron mechanism. A relativistic electron population with
a power law energy spectrum 
will produce a flux density
, for an optically thin
source with uniform magnetic field 
. In this case, the degree of
linear polarization will be approximately (e.g. Pacholczyk 1970):
where p is related with the observed spectral index by 
. In
order to account for the polarization reported by Junkes
et al. (1993) a
random magnetic field 
must be present in the source. Then,
the degree of polarization becomes:
Using the spectral indices presented in this paper we find
in the Double Inner Lobes and in the Northern
Middle Lobe. In the steepest regions of the Giant Lobes the random field
is 
.
Several models have been invoked to explain the large-scale radio appearence of Cen A (e.g. Blandford & Rees 1974; Kellerman & Pauliny-Toth 1981, van Albada et al. 1982; Quinn 1982). The recent work by Junkes et al. (1993), which established that the Nothern and Southern Giant Lobes are linked by a bridge of continuum emission, together with the nonthermal spectral index distribution displayed in Fig. 2 (click here) seem to support the expanding jet model with induced shocks as a method for powering the extended radio source. This model is also supported by evidence of ongoing activity in the nucleus (Kellermann 1974; Morganti et al. 1992; Botti et al. 1993), and by X-ray, optical and radio observations of the inner jet of the source (Schreier et al. 1979; Feigelson et al. 1981; Burns et al. 1983; Haynes et al. 1983; Meier et al. 1989).
Summarizing, the large-scale radio emission of Cen A seems to agree with the picture of an active nucleus with a jet forming a relatively small angle with the line of sight. This jet, which can be directly observed at VLA scales, would be responsible for the formation of the Northern Inner and Middle Lobes when it interacts with the interstellar and intergalactic medium, respectively. The Northern Giant Lobe would be the cocoon associated to the Middle Lobe. Finally, the southern Inner and Giant Lobes would be produced by a counter jet and, consequently, they would be situated farther from the observer than the northern lobes. This fact seems to be reflected by the lower degree of polarization observed in the southern components.
Acknowledgements
We thank P. Benaglia for his critical reading of the manuscript. This work was partially supported by CONICET and the Universidad Nacional de La PLata.