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Session 68 - Disk Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[68.11] The Nature of Arms in Spiral Galaxies: Symmetries and Asymmetries

M. S. d. R. Alvarez, E. Brinks (Dep. de Astronom\'\ia, IFUG), M. S. d. R\'\io, J. Cepa (I.A.C.)

A Fourier analysis of the intensity distribution in the plane of nine spiral galaxies is performed. Seven have well--defined arms (classes 12 and 9) and two have intermediate type arms (class 5) according to the arm classification of Elmegreen amp; Elmegreen (1987, ApJ, 314, 3). The galaxies studied are NGC 157, 753, 895, 4321, 6764, 6814, 6951, 7479 and 7723. For each object Johnson B band images are available which are decomposed in angular components, for different angular periodicites. No assumption is made, a priori, about the form of the arms. The base function used in the development is a logarithmic spiral. The main result obtained with this method is that the dominant component (or mode) usually changes at co-rotation. In some cases, this change to a different mode only persists for a short range around co-rotation, but in other cases the change is permanent. The agreement between pitch angles found with this method and by fitting logarithmic spirals to mean arm positions (del R\'\io amp; Cepa 1996, ApJ, submitted) is good, except for those cases where bars are strong and dominant. Finally a comparison is made with the ``symmetrization'' method introduced by Elmegreen, Elmegreen amp; Montenegro (1992, ApJS, 79, 37), which also shows the different symmetric components.

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Program listing for Wednesday