The large scale structure of the galactic center at low radio frequencies

A region about 10◦ around the galactic center (G.C.) has been studied in surveys available between 19.7 and 408 MHz. The maps show a trough near the G.C. sided by two intense emission peaks. Interpreting the observations as produced by an HII region that absorbs the radiation from bright nonthermal sources we propose a model consisting of four sources concentric with the G.C. and embedded in a diffused nonthermal background. Two sources, the broad and the narrow, are non-thermal and we determine their temperature spectrum (indices −2.7 and −2.4, respectively), the third source is an HII region for which we compute the emission measure, and the fourth source is Sgr A complex. The broad source is prominently seen below 408 MHz while the presence of the narrow source is reported here for the first time.