Structural and Photometric Properties of Extragalactic Stellar Spheroids
Wirth, Allan
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https://hdl.handle.net/2142/70661
Description
Title
Structural and Photometric Properties of Extragalactic Stellar Spheroids
Author(s)
Wirth, Allan
Issue Date
1982
Department of Study
Astronomy
Discipline
Astronomy
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Astronomy and Astrophysics
Abstract
After an overview of the science of galaxy morphology, a new classification for galactic form, based upon the Yerkes system, is presented. This classification scheme is then applied to the study of the role of environment in determining the nature of galaxy populations. By examining the distribution of galaxy types in 11 clusters and groups a number of new facts are brought to light: The relation between spiral fraction and local density found by Dressler is confirmed. However, the elliptical galaxy fraction, in groups they contain at least one elliptical, is found to be independent of density. The dwarf galaxy populations in and out of the cluster environment are shown to be very different. Two distinct families of low luminosity spheroids are identified, and their properties compared.
Next the photometric properties of spheroids are probed using multi-color surface photometry. By extending this photometry to very low surface brightness levels and to a large and varied sample of spheroids, the following are found: First, pure spheroids of all luminosities are well mixed, showing small color gradients out to large radii. Second, the mean metallicity in ellipticals is near solar. Third, only disc dominated systems show metal poor halos. Constraints upon the nature of massive halos are presented. Using the classification system defined here, a sample of pure spheroids is created and the relationships between the global properties of these systems are examined. It is shown that the color-magnitude relation for late-type galaxies is due primarily to a systematic variation in galaxy type included in other samples and is only weakly observed in true ellipticals. The color is shown to be strongly correlated with velocity dispersion.
In the last section these observations are compared with the predictions of theory. It is found that no one theory of galaxy formation can adequately explain all the observed properties of spheroids. Further, even hybrid models cannot provide explanations for the relations seen between the global properties of ellipticals. Lastly, a few lines of observational research are described.
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