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Astron. Astrophys. 359, 932-940 (2000)


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The detection of spiral arm modulation in the stellar disk of an optically flocculent and an optically grand design galaxy

I. Puerari 1, D.L. Block 2, B.G. Elmegreen 3, J.A. Frogel 4 and P.B. Eskridge 4

1 Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México
2 Department of Computational and Applied Mathematics, University Witwatersrand, Private Bag 3, WITS 2050, South Africa
3 IBM Research Division, T.J. Watson Research Centre, P.O. Box 218, Yorktown Heights, NY 10598, USA
4 Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210-1173, USA

Received 28 March 2000 / Accepted 11 May 2000

Abstract

Two dimensional Fourier spectra of near-infrared images of galaxies provide a powerful diagnostic tool for the detection of spiral arm modulation in stellar disks. Spiral arm modulation may be understood in terms of interference patterns of outgoing and incoming density wave packets or modes. The brightness along a spiral arm will be increased where two wave crests meet and constructively interfere, but will be decreased where a wave crest and a wave trough destructively interfere. Spiral arm modulation has hitherto only been detected in grand design spirals (such as Messier 81). Spiral arm amplitude variations have the potential to become a powerful constraint for the study of galactic dynamics. We illustrate our method in two galaxies: NGC 4062 and NGC 5248. In both cases, we have detected trailing and leading m=2 waves with similar pitch angles. This suggests that the amplification mechanism is the WASER type II. In this mechanism, the bulge region reflects (rather than refracts) incoming waves with no change of pitch angle, but only a change of their sense of winding. The ratio between the amplitudes of the leading and the trailing waves is about 0.5 in both cases, wherein the higher amplitude is consistently assigned to the trailing (as opposed to leading) mode. The results are particularly significant because NGC 5248 is an optically grand design galaxy, whereas NGC 4062 is optically flocculent. NGC 4062 represents the very first detection of spiral arm modulation in the stellar disk of an optically flocculent galaxy.

Key words: galaxies: spiral – galaxies: structure – galaxies: kinematics and dynamics – galaxies: individual: NGC 4062 – galaxies: individual: NGC 5248 – methods: numerical

Send offprint requests to: I. Puerari (puerari@inaoep.mx)

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000

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