Astron. Astrophys. 355, 863-872 (2000)

4. Conclusions

We have presented and discussed the integrated flux density spectrum of the large-scale Cen A components. To obtain the spectra we have resorted to three types of data: i) our own observations at 45 MHz, ii) results deduced by us from data found in the literature, and iii) results obtained by other authors. In this way we have studied the spectra in extended frequency ranges, resolving controversies about the spectral index of the Whole Source and presenting, for the first time, the spectra of the individual Giant Outer Lobes.

The conclusions are as follows:

1. We have obtained the total integrated spectrum of the whole of Cen A using data of types i, ii and iii. At 45 MHz we have used our own observations and determined a flux density of Jy. The spectrum between 10 and 4750 MHz is fitted well by a power law of index . This result belies a spectral break reported by other authors in the past.

2. Using data of types ii and iii we have determined, for the first time, the spectra of the individual Outer Giant Lobes between 406 and 4750 MHz. The North and South lobes fit the mean values with power laws of indices and , respectively. That is, the spectral indices are practically equal.

3. We have obtained the individual spectra of the Inner Lobes using data of type iii. Between 80 MHz and 43 GHz, the Northeastern and Southwestern lobes' fluxes are fitted well by a power law of index . This results confirm that of Slee et al. (1983) obtained over a smaller frequency range.

4. We have calculated the luminosities of the Whole Source, GLs and ILs, assuming that their spectra, determined from mean values, follow power laws between 4.7 MHz and 43 GHz. A distance of 3.5 Mpc was adopted. We have found that the GLs are four times more luminous than the ILs and that the luminosity ratio GLS/GLN is 1.20, while the corresponding ratio ILSW/ILNE is 0.68. The total luminosity of Cen A is 23.9 erg s^-1.

5. Since the GLs contribute 73% of the luminosity of the Whole Source we would expect the index of the GLs to be very close to that of the Whole Source. The errors in the fit of the GLs make this expectation possible.

6. The similarities between the spectral indices of the GLs and ILs support the hypothesis that the structure was formed by two successive ejections from the same parent population of relativistic electrons, or from two different populations generated at the time of ejection with the same energy spectrum. Differences in the luminosities between the ILs and between the GLs could be attributed to differences in electron ejection densities. The fact that there is no noticeable steepening at the high frequency end could indicate that the object is young, or that there is reacceleration or continuous ejection.

7. Assuming only synchrotron and inverse Compton losses, the GLs and ILs are younger than and years, respectively.

8. We have shown that Cen A supports the P- rather than the z- correlation.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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