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Astron. Astrophys. 340, 1-20 (1998)


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The nature of arms in spiral galaxies

III. Azimuthal profiles

M.S. del Río 1, 2 and J. Cepa 2, 3

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 13 March 1998 / Accepted 24 August 1998

Abstract

In this paper we analyse the structure of a small sample of galaxies using a set of CCD images in standard photometric bands presented in a previous paper (del Río & Cepa 1998a, hereafter Paper II). The galaxies are NGC 157, 753, 895, 4321, 6764, 6814, 6951, 7479 and 7723, and the selected bands were B and I. Seven galaxies are grand design, i.e. they have two long and symmetric arms, second in the classification of Elmegreen & Elmegreen(1987), and are the best laboratories for testing the predictions of the spiral density wave (SDW) theory. Two of the galaxies have intermediate arms, i.e., they are not so well defined. They are selected to compare the results with those found in the grand design spirals. Using the method of analyse the azimuthal flux profiles presented by Cepa(1988) and Beckman & Cepa (1990) (hereafter Paper I) and assuming that star formation is triggered by a spiral density wave, we look for evidence of the existence of a corotation radius, as predicted by the SDW theory. We have determined the corotation radius in all but two grand design galaxies, and, tentatively, in the other four. Galaxies with very weak arms (such as NGC 753 and NGC 6951) or arms which are not well defined (such as NGC 6764 and NGC 7723) present difficulties when employing the azimuthal profile method, but even in these cases, the method is powerful enough to give a good estimate of the value of corotation, which must then be confirmed (or discarded) by other independent methods (del Río & Cepa 1998b, hereafter Paper IV).

Key words: galaxies: spiral – galaxies: structure

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

© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998

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