EDP Sciences
Free access
Issue A&A
Volume 490, Number 3, November II 2008
Page(s) 1005 - 1017
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:200810590
Published online 23 September 2008

A&A 490, 1005-1017 (2008)
DOI: 10.1051/0004-6361:200810590

High-resolution radio continuum survey of M 33

III. Magnetic fields
F. S. Tabatabaei1, M. Krause1, A. Fletcher2, and R. Beck1

1  Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: tabataba@mpifr-bonn.mpg.de
2  School of Mathematics and Statistics, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK

Received 14 July 2008 / Accepted 27 August 2008

Aims. We study the magnetic field structure, strength, and energy density in the Scd galaxy M 33.
Methods. Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across M 33. We fit a 3D model of the regular magnetic field to the observed azimuthal distribution of polarization angles. We also analyze the spatial variation of depolarization across the galaxy.
Results. Faraday rotation, measured between 3.6 and 6.2 cm at an angular resolution of 3$\arcmin$ (0.7 kpc), shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M 33 at 3.6 cm is polarized. High degrees of polarization of the synchrotron emission (>$20\%$) and strong regular magnetic fields in the sky plane ($\simeq$$6.6~\mu$G) exist in-between two northern spiral arms. We estimate the average total and regular magnetic field strengths in M 33 as $\simeq$6.4 and 2.5 $\mu$G, respectively. Under the assumption that the disk of M 33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3 to 5 kpc radius.
Conclusions. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m=0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M 33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M 33's rotation curve. Generation of interstellar magnetic fields from turbulent gas motion in M 33 is indicated by the equipartition of turbulent and magnetic energy densities.

Key words: galaxies: individual: M 33 -- radio continuum: galaxies -- galaxies: magnetic fields -- galaxies: ISM

© ESO 2008

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.

Editor-in-Chief: T. Forveille
Letters Editor-in-Chief: J. Alves
Managing Editor: C. Bertout

ISSN: 0004-6361 ; e-ISSN: 1432-0746
Frequency: 12 volumes per year
Published by: EDP Sciences

Mirror sites: CDS | EDP Sciences
  RSS feeds
© The European Southern Observatory (ESO)