The synthesis of composite spectra : a study of four globular clusters of the galaxy, five globular clusters of M31, and the nucleus of NGC 205

Citation

Christensen, Clark G. (1972) The synthesis of composite spectra : a study of four globular clusters of the galaxy, five globular clusters of M31, and the nucleus of NGC 205. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/VTAK-FR10. https://resolver.caltech.edu/CaltechETD:etd-05212008-111500

Abstract

A spectral synthesis technique similar to that of Spinrad and Taylor (1971) has been applied to the integrated light of globular clusters M5, M13, M15, and M92 of the Galaxy; H12, H55, H140, B282, and MIV of M31; and to the nucleus of the elliptical galaxy NGC 205. The derived stellar contents of the four local clusters have been compared with color-magnitude diagrams and luminosity functions based on studies of individual stars within these systems. Results agree.

The technique yields information on both the metal content of clusters and their horizontal-branch colors and thus is useful for extracting the two parameters which apparently describe galactic globular clusters.

Principal results of the study of the six extragalactic systems are: (1) The clusters H12 and H140 have similar metal contents and similar luminosity functions except along their horizontal branches where H12 is concentrated to the red, resembling 47 Tuc, and H140 is rather blue, resembling M5. This result suggests that M31 clusters belong to at least a two-parameter family as do clusters in the Galaxy. (2) H55 and B282 have absorption features too strong to be artificially reproduced with a mixture of population I stellar spectra of normal feature strengths. This confirms a similar result by Spinrad and Schweizer (1972) for B282. (3) The strong-lined cluster H55 is a halo object. This corroborates van den Bergh's (1969) discovery of strong-lined halo clusters in M31. (4) The metal-richness of the stellar contents of the clusters correlates well with van den Bergh's line index L. This correlation strengthens his result that M31 globular clusters are generally more metal-rich than their galactic counterparts. (5) No firm evidence is found for the hot blue stars inferred to be present in B282 by Spinrad and Schweizer. (6) The nucleus of NGC 205 is composed of mild population II stars like clusters H12 and H140. However its color-magnitude distribution is unlike that of any known globular cluster. V-light contributions are about sixty per cent from its main sequence and only three per cent from its giant branch. The domination by dwarfs is greater by a factor of two than in the M81, M32, and M31 nuclear models of Spinrad and Taylor (1971). The apparent explanation is a different luminosity function.

There is some evidence for dispersion in the initial luminosity functions of clusters. The best models for the metal-rich clusters H55 and B282 have about one-half more V-light arising from their main sequences than the other clusters. The giant branch of M5 appears to contribute about one-third more V-light than giant branches of other clusters studied. There is some independent evidence for an abnormally heavily populated giant branch in M5.

Thesis Files