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

1. Introduction

The University of Chile has just finished a 45-MHz sky survey, covering from the south pole up to , with a transit array having an angular resolution of x (Alvarez et al. 1997). In the course of this work we noticed that the point with the highest brightness temperature in the region around the galactic center is not at the position , , as expected from higher frequency surveys, but it is at , . This apparent shift of the brightest point led us to examine the large scale structure of a region about x centered at the G.C., in maps made at frequencies equal or lower than 408 MHz. These surveys are listed in Table 1.

Table 1. Low-frequency surveys covering the galactic center area

In what follows we will designate a point at galactic coordinates , as (, ). The maps at 85.7, 34.5, 30.9, 29.9 and 19.7 MHz show a trough near (0, 0), first noticed by Mills (1956) who saw "an extended bright emission region [that] stretches along the galactic plane for about 5 or 6 with two maxima about 2 apart, more or less symmetrically situated with regard to the centroid of the object, which is practically coincident with the source NRL5". This source had been suggested as the galactic nucleus by Haddock et al. (1955). Mills further suggested that such a structure could be "explained in terms of an HII region in front, or partly in front, of an elongated non-thermal source. With an electron temperature of about 10000 K, the HII region would appear in absorption against the much brighter background radiation, producing maxima in emission on either side, similar to those observed". Although Mills observations have been mentioned by several authors, e.g. Cooper & Price (1964), Burke (1965), Little (1974), they have not been further investigated. The purpose of this paper is to test some of these ideas with new observations at other frequencies.

In a first approximation we have tried a preliminary model whose failure suggested a more elaborate one which is analyzed in this work.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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