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

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1. Introduction

Centaurus A (IAU 13S4A, NGC 5128, B1322-428, J1325 -4303) is the strongest extragalactic radio source in the southern hemisphere and, being the closest radio galaxy to us, its structure can be studied in detail. It is an extended, complex and fairly symmetric source that exhibits two Giant Outer Lobes (GLs), the northern one (GLN) and the southern one (GLS), that span declinations between approximately [FORMULA] and [FORMULA]. Closer to the center are two smaller Inner Lobes (ILs), situated to the northeast and southwest of the center, which we will designate as ILNE and ILSW, respectively. Approximately to the north of the ILNE is the Northern Middle Lobe, which has no symmetric counterpart in the south. The large feature between approximately [FORMULA] and [FORMULA] has been called the Northern Loop in the literature (Junkes et al. 1993). In the nuclear region are the nucleus, the jet, the counter jet and the knots. Good descriptions on Cen A morphology are found in Burns et al. (1983) and in a recent review by Israel (1998). Fig. 1 presents a 4.75 GHz map showing clearly all the large scale features of this radio galaxy. Some authors refer to the ILs plus the region closer to the center as the central source or as the central component . Also some authors refer to the GLs as the extended source or simply as the outer lobes . The morphological definitions we will use are as follows:

  1. the Whole Source is Cen A in its entire extension,

  2. the Inner Lobes (ILs) follow the usual definition, where ILNE indicates the inner lobe in the NE direction and ILSW in the SW direction, respectively,

  3. the region internal to the ILs will be called Nuclear Region (NR),

  4. the NR plus the ILs will be designated as the Central Region (CR),

  5. the Giant Northern Lobe (GLN) comprises the region north of the center ([FORMULA]) less the corresponding part of the CR; thus the GLN includes the Northern Middle Lobe and the Northern Loop,

  6. the Giant Southern Lobe (GLS) comprises the region south of the center less the corresponding part of the CR.

[FIGURE] Fig. 1. Map of Centaurus A at 4.75 GHz (Junkes et al. 1993), smoothed to a resolution of [FORMULA]. The temperature contours are not indicated since the purpose of the figure is to show the morphology of the object. All large scale components are clearly visible: the Northern Loop ([FORMULA]), the Northern Middle Lobe ([FORMULA]), the Central Region ([FORMULA]), the GLS ([FORMULA], and the GLN ([FORMULA])(see text)

In what follows whenever we refer to a spectrum we will mean an integrated flux density spectrum. In the radio range the source has been observed between 4.7 MHz and 43 GHz by several authors; however, few of them have studied the spectrum of the Whole Source and none has attempted to investigate that of the individual GLs. The spectrum of the individual ILs has been well studied.

There are a number of discrepancies regarding the spectral index of the Whole Source. The first to investigate the spectrum of Cen A were Roman & Haddock (1956), although they failed to explicitly state the components of Cen A's emission considered in their study. Between 60 and 3200 MHz they found a discontinuity that resembled two straight segments with approximately the same index ([FORMULA], with [FORMULA]) but shifted in frequency, thus forming a sort of plateau around 500 MHz. A different result was obtained, in a seminal paper on the subject, by Cooper et al. (1965). These authors found a strong, positive curvature in the spectrum (i.e. a spectrum with negative index, that steepens with decreasing frequency) between 19.7 and 1410 MHz. The discrepancies between these two works emphasize that well-defined spectra of the whole source and of its components are important in studying the origin and evolution of the object. The first purpose of this paper is an attempt to resolve these discrepancies.

In the literature there is a great variety of values for flux densities and spectral indices for the different components of Cen A. There is even confusion about the nomenclature, particularly regarding the smaller components. Partly for this reason we have made a careful and exhaustive study of the published material related to the Inner Lobes. This has also been necessary in order to compare the parameters of these lobes with those of the Whole Source and Giant Outer Lobes. Since we are interested in the study of the flux density of the large scale structures of Cen A we have made an attempt, for the first time, to determine the spectrum of the individual Giant Lobes. This is the second purpose of this paper.

To investigate the spectrum of the large scale components of Cen A we use our own observations at 45 MHz, the measurements at other frequencies found in the literature, and the determinations made by us from the data published by other authors. We compare the spectrum of the different components and briefly discuss them in relation to their origin and evolution.

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Online publication: March 21, 2000