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Astron. Astrophys. 362, 910-920 (2000)


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Detection of spiral magnetic fields in two flocculent galaxies *

J. Knapik 1, M. Soida 1, R.-J. Dettmar 2, R. Beck 3 and M. Urbanik 1

1 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
2 Astronomisches Institut, Ruhr-Universität-Bochum, 44780 Bochum, Germany
3 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 14 June 1999 / Accepted 2 August 2000

Abstract

According to the classical axisymmetric dynamo concept, differentially rotating galaxies which lack organized optical spiral patterns and density wave flows should still have spiral magnetic fields with a substantial radial component. To check this hypothesis we observed two flocculent spirals, NGC 3521 and NGC 5055, in the radio continuum (total power and polarization) at 10.55 GHz with a resolution of [FORMULA]. A search for traces of optical spiral patterns has also been made by observing them in the H[FORMULA] line and by filtering their available blue images.

NGC 3521 and NGC 5055 were found to possess a mean degree of magnetic field ordering similar to that in grand-design spirals. However, the polarized emission fills the central region of NGC 5055 while a minimum of polarized intensity was observed in the inner disk of NGC 3521. This can be explained by a more uniform star formation distribution in the centre of NGC 3521, while a higher concentration of star-forming activity in the nuclear region and in the rudimentary spiral "armlets" of NGC 5055 leaves broader interarm regions with unperturbed regular magnetic fields. Both galaxies possess regular spiral magnetic fields with a radial component amounting to 40% - 60% of the azimuthal field. The use of beam-smoothed polarization models demonstrates that this result cannot be produced by limited resolution and projection effects. Furthermore, a large magnetic pitch angle cannot be entirely due to the influence of rudimentary spiral-like features visible in our H[FORMULA] and enhanced optical images. Thus it appears that the dynamo process is responsible for the radial magnetic field in flocculent galaxies. The measured radial magnetic field component as compared to the azimuthal one is even stronger than predicted by a classical turbulent dynamo which provides arguments in support for modern, non-standard dynamo concepts.

Key words: polarization – galaxies: individual: NGC 3521, NGC 5055 – galaxies: magnetic fields – galaxies: spiral – radio continuum: galaxies

* Partly based on observations obtained at Lowell Observatory, Flagstaff, AZ (USA)

Send offprint requests to: J. Knapik (knapik@oa.uj.edu.pl)

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000

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