Microwave Spectra of OH in the Direction of the Galactic Center.

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Microwave Spectra of OH in the Direction of the Galactic Center. Gundermann, E. J. ; Goldstein, S. J., Jr. ; Lilley, A. E. Abstract Using the Harvard 60-ft radio telescope and a maser radiometer, we have studied the microwave OH line complex in 19 directions toward the galactic center. In each direction investigated, the OH spectra appear exclusively in absorption and display multiple components of unusually large half-widths. The absorption spectra were found in the range 111=358.5 to 4~, (b"-00), where typical profiles contain three maj or components ranging in velocity from +140 to -140 km/sec with characteristic half-widths of approximately 80 km/sec. The velocity resolution of the maser radiometer was 15 km/sec. Three spectra of the OH line complex have been observed for each galactic direction, viz., the F=2 ~2 (1667 Mc/sec), F=1~1 (1665 Mc/sec),andthe F=2~1 (1612 Mc/sec) transitions. for these transitions, the predicted ratios for optically thin lines are 9:5:1 at 1667, 1665, and 1612 Mc/sec. A major result is that the measured line ratios are invariably smaller for each of the observed positions. Several explanations have been offered for the anomalous ratios; the most simple is large optical depths. The hypothesis of line saturation has been tested in detail on a number of individual absorption components. The ratio of observed antenna temperature minima at 1667 and 1665 Mc/sec and the assumption that the theoretical opacities ratios apply allow the calculation of an antenna temperature minimum at 1612 Mc/sec. In 14 cases, we compared the calculated and observed temperatures at 1612 Mc/sec. Within the accuracy limits imposed by radiometer noise, 12 cases satisfy the line saturation hypothesis. This general result suggests that the anomalous line ratios are due primarily to large optical depths (~ 1-2) in directions toward the galactic center. The OH velocity longitude diagram has been compared with similar diagrams for neutral hydrogen. Although the total velocity ranges are comparable, nevertheless, in the galactic central regions there is very little correlation of OH and hydrogen features, and no evidence of the nuclear disk in the OH data. However, considering the great difference in state temperatures of hydrogen and OH, there are effects which could mask similarities in the hydrogen and OH spectra. Publication: The Astronomical Journal Pub Date: June 1965 DOI: 10.1086/109734 Bibcode: 1965AJ.....70..321G full text sources ADS |