Astron. Astrophys. 327, 569-576 (1997)

4. Discussion

A model compatible with the observations consists of four components that for simplicity have been assumed spherical and concentric with the G.C. Table 4 presents the parameters of the revised model. It is difficult to evaluate the errors in temperature as they depend on factors such as calibrations, zero levels, scatter in the measurements, beam smoothing, and others features of the surveys not well known. The accuracy in temperature estimated by the authors of the surveys at 408, 85.7 and 45 MHz are better than 10, 15 and 10%, respectively. In the case of the models presented in this work an additional error derives from fitting gaussians to the data. In order to estimate the goodness of fit we studied the correlation between the measured temperatures and those given by the fit curve. The correlation coefficient for the four fits is equal or better than 0.95, so the corresponding error in temperature shoul be small. We consider that the inhability of the beam to fully resolve the trough at 45 MHz is the main cause of uncertainty in temperature at this frequency. Regarding Sgr A and the HII region, it has been mentioned that the temperatures of the former were taken from Pedlar et al. (1989), assuming it to be a point source, while the adopted parameters for the latter are: electronic temperature (Matthews et al. 1973 a, b), angular size 1.5 x 1.0, x , (Jones and Finlay, 1974), and emission measure 3.6 10⁴ pc cm^-6, derived by us.

Table 4. Parameters of the revised model

The temperature spectrum of the broad source exhibits an index of -2.7, confirming its non-thermal nature. This source has been noticed in the literature as a big bump in temperature profiles taken along the galactic equator, and centered approximately in the G.C.; however this feature has not been thought of as being an individual entity. Several surveys show it with temperature contours very close to ellipses with the major axis lying approximately along the galactic equator. In the 45 and 30-MHz surveys the axial ratio is 1.7 and the center is shifted towards positive longitudes, 1 . This low frequency shift could be explained if the western peak were observed weakened by the HII region whose absorbing effect extends to the western edge of the broad source as the frequency lowers. This hypothesis would find support from the fact that, as the frequency decreases, the western peak is the one that gets weaker and that its position is the one that changes the most. (See Table 2). Assuming that the broad source is actually centered at (0, 0 ), the above explanation would require a fairly large HII region, large enough to shift by 2 the center of a 8 x 5 structure, while maintaining the symmetry of the observed contours. Even though this is a likely scenario, the apparent displacement of the center of symmetry of the broad source could be real. In this connection it is interesting to recall that the centroid of the G.C. molecular gas has also been found displaced towards positive longitudes, (Bally 1996).

Because it is the brightest object in the radio Galaxy and because of its large size it seems reasonable, in spite of the 2 shift, to think that it is associated with the nuclear region of the Galaxy. In any case, the broad source is such a large object that it does not fit easily into the known types of non-thermal galactic radio sources. Therefore, we suggest it is a major component of our Galaxy and in analogy with the optical structure we would like to define it as the radio bulge.

The narrow source is a new finding. Evidence for its emission is seen in the 408-MHz data, however at lower frequencies it is masked by the thermal absorption. A precedent for the existence of this source can be found in the work of Little (1974) who failed to account for all the radiation received at 408 MHz. We have obtained the temperature spectrum of this source which shows an spectral index of -2.4, indicative of its non-thermal nature. We could speculate that the narrow source, as seen at low frequencies, could actually be the 7 nuclear halo studied by Pedlar et al. (1989). However, below 408 MHz the computed absorption by the HII region falls short to explain the steep turn over observed in the spectrum, which we have assumed inherently straight.

With the basic physical parameters of the HII region adopted in this work we have computed an emission measure of 3.6 10⁴ pc cm^-6 which is in excellent agreement with the one and only determination found in the literature (Matthews et al. 1973a, b).

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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