Astron. Astrophys. Suppl. Ser. 134, 333-358

The nature of arms in spiral galaxies

IV. Symmetries and asymmetries

M.S. del Río^1,2 - J. Cepa^2,3

Send offprint request: M.S. del Río
e-mail: sole@cibeles.astro.ugto.mx

1 - Departamento de Astronomía, IFUG, Universidad de Guanajuato, 36000 Guanajuato, Mexico
2 - Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
3 - Departamento de Astrofísica, Facultad de Física, Universidad de La Laguna, E-38071 La Laguna, Tenerife, Spain

Received April 24; accepted August 24, 1998

Abstract:

A Fourier analysis of the intensity distribution in the planes of nine spiral galaxies is performed. In terms of the arm classification scheme of Elmegreen & Elmegreen (1987) seven of the galaxies have well-defined arms (classes 12 and 9) and two have intermediate-type arms (class 5). 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 into angular components, for different angular periodicities. No a priori assumption is made concerning the form of the arms. The base function used in the analysis is a logarithmic spiral. The main result obtained with this method is that the dominant component (or mode) usually changes at corotation. In some cases, this change to a different mode persists only for a short range about corotation, 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 Río & Cepa 1998b, hereafter Paper III) 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 & Montenegro (1992, hereafter EEM), which also shows the different symmetric components.

Key words: galaxies: spiral -- galaxies: structure of