Spectral Indices of 60 Discrete Sources.

Abstract

Flux densities of 60 discrete sources were measured with a 60-ft reflector and maser radiometer at 1422.4 Mc at Harvard Observatory. The sources have galactic latitudes numerically greater than 10 deg, and were selected from the 85.5-Mc survey by Mills, Slee, and Hill and from the 159-Mc Third Cambridge Survey of Edge et al. The observations consisted of at least two scans in right ascension and two scans in declination across each source; thus the position at 1422.4 Mc could be compared to that reported at lower frequency. In only three cases was the difference in positions greater than 7 minutes of arc. Calibration was obtained from observations of Cas A and the adoption of Findlay's flux density. Spectral indices obtained for these sources have a narrower range than has been previously reported and seem independent of flux density. An explanation is given for the apparent correlation between flux density and spectral index found by other observers. The sources that are identified with galaxies in the Shapley-Ames catalogue have indices confined to the central part of the spectral index distribution, while those identified with more distant anonymous galaxies have a wider distribution of indices. This result is in conflict with recent findings of Heeschen. The sources listed as definitely extended by Mills, Slee, and Hill also have indices near the center of the distribution. If sources have a constant index with respect to frequency and if either the present observations or those at 960 Mc by Harris and Roberts are substantially correct, then the Third Cambridge flux densities are consistently low or the Mills-Slee-Hill flux densities are consistantly high. This applies even to sources whose angular sizes, measured by Moffet at 960 Mc suggest that an error due to angular size was not made in the observations at 159 Mc. The research program at Harvard is sponsored by the National Science Foundation.