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One of the main reasons for pursuing a gravitational as opposed to a magnetic theory is that there exist galaxies which seem devoid of gas and yet show unmistakable structure. These galaxies are among those classified as SBO 2/3 to SBa(r), and I shall, after Curtis, call them theta galaxies. Examples are: NGC 1512, 1291,4693, and 2859. The theta galaxies gradually merge into ringed bar galaxies, which in turn merge into ordinary spirals.
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Agris J. Kalnajs (1973) studied this question.
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