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

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2. The data

Some of the observations listed in Table 1 are of limited use for our purpose because: a) high resolution observations have been obtained with aperture synthesis, loosing the low spatial frequencies and preventing the study of large or medium scale structure; b) the 29.9-MHz map by Jones & Finlay (1974) is a composite of original synthesis observations to which the galactic background at 30 MHz, taken from Mathewson et al. (1965), was added. The method used to combine the maps is based on several assumptions and it is subjected to errors; c) The 85 and 150-MHz data from Wielebinski et al. (1968), correspond to background surveys from which point sources were removed (Wielebinski, private communication), so possible peaks in the center area may have been lost; finally, d) the 19.7-MHz data from Shain et al. (1961) suffer from severe and widespread absorption. Because of these limitations, and except for restricted use of the 29.9 and 30-MHz data, our analysis has been based only on the surveys at 45, 85.7 and 408 MHz.

The two maxima noticed by Mills near the G.C. at 85.7 MHz (Mills 1956, Fig. 1) are centered approximately at galactic coordinates (-0.7[FORMULA] -0.3[FORMULA] ) and (1.2[FORMULA], 0.0[FORMULA] ), and we will define them as the western and eastern peaks, respectively. These peaks are seen in the survey at the same frequency made by Hill et al. (1958), and they appear as well at 29.9 MHz (Jones & Finlay 1974, Fig. 2f). At 45 MHz the eastern peak, at (2.1[FORMULA], 0.1[FORMULA] ), is dominant while the western peak, at about (-1.5[FORMULA], 0.5[FORMULA] ), is not completely resolved (Alvarez et al. 1997). The maxima are clearly seen also in the 34.5 and 19.7-MHz maps, however in the 30-MHz survey they are washed out because of its low resolution (11[FORMULA] ). The 30.9-MHz synthesis map has too high a resolution (13.0[FORMULA] x 11.1[FORMULA] ) and shows abundant small scale structure so the two peaks are not recognizable. The 408-MHz map by Haslam et al. (1982) shows a single peak centered at (0[FORMULA], 0[FORMULA] ), while in the high resolution map by Green (1974), at the same frequency, there is no indication of any of the peaks. From this description we may conclude that the two maxima begin to appear somewhere between 408 and 85 MHz. Between the peaks there is a trough which is clearly seen at 85.7, 34.5, 30.9, 29.9 and 19.7 MHz. The position and the estimated uncertainties of these three features at different frequencies are shown in Table 2.


Table 2. Positions of peaks and trough near the galactic center

The position of the trough is not very sensitive to frequency and at 45 MHz it is more uncertain because of beam smoothing. The eastern peak is somewhat more sensitive to frequency, specially in longitude, while the western peak shows large changes in both coordinates, specially in latitude. The last column of Table 2 shows also the angular distance between the centers of the peaks.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998