An Aperture Synthesis Survey of the Galactic Plane

Abstract

The results of a survey already published have been used to construct contour maps and ruled surface diagrams of the brightness temperature at 29·9 MHz near the galactic plane between I = 225° and 30°. The angular resolution was 0°· 8 at the zenith, and the range of zenith angles involved was ± 30°. Restoration of the background was achieved with the aid of a low resolution filled-aperture survey carried out by others. The brightness temperature scale was calibrated absolutely. The optical depth of the Galaxy in directions within 40° of latitude from the centre has been estimated by a method which relies only on the shapes of brightness temperature profiles and not on absolute temperature calibrations. If an electron temperature is assumed, r.m.s. electron densities can be deduced. The average value of the disc emissivity at 29·9 MHz and the value of its spectral index have been calculated from brightness temperature profiles observed at a number of different frequencies, calibrations being required for these purposes. About 29 discrete absorption regions have been observed and identified with optically observed HII regions, and the fact that these are all nearer than 4 kpc permits a choice between kinematic distances in two cases. The Carina nebula and RCW 108 lend themselves to the measurement of local emissivities, and values of these together with their implications have already been published. A number of previously unlisted nonthermal sources have been observed, many of which are objects of low surface brightness and probably are supernova remnants.