EDP Sciences
Free access
Volume 405, Number 2, July II 2003
Page(s) 513 - 524
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030628

A&A 405, 513-524 (2003)
DOI: 10.1051/0004-6361:20030628

Magnetic fields and ionized gas in the local group irregular galaxies IC 10 and NGC 6822

K. T. Chyzy1, J. Knapik1, D. J. Bomans2, U. Klein3, R. Beck4, M. Soida1 and M. Urbanik1

1  Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
2  Astronomisches Institut der Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
3  Radioastronomisches Institut der Universität Bonn, Germany
4  Max-Planck-Institut für Radioastronomie, Postfach 2024, 53010 Bonn, Germany

(Received 28 February 2003 / Accepted 24 April 2003)

We performed a high-sensitivity search for galaxy-scale magnetic fields by radio polarimetry at 10.45 GHz and 4.85 GHz with the Effelsberg 100 m radio telescope, accompanied by H $\alpha$ imaging, for the two Local Group irregular galaxies IC 10 and NGC 6822. Their star-forming bodies are small and rotate slowly. IC 10 is known to have a very high star-forming activity, resembling blue compact dwarfs, while NGC 6822 has a low overall star-formation level. Despite very different current star formation rates, our H $\alpha$ imaging revealed a large web of diffuse H $\alpha$ filaments and shells in both IC 10 and NGC 6822. Some of them extend far away from the galaxy's main body. The total power emission of both objects shows bright peaks either at the positions of optically strong star-forming clumps (IC 10) or individual $\ion{H}{ii}$ regions or supernova remnants (NGC 6822). However, in both cases we detect a smoothly distributed, extended component. In IC 10 we found clear evidence for the presence of a diffuse, mostly random magnetic field of $\simeq$14 $\mu$G strength, probably generated by a fluctuation dynamo. One of the H $\alpha$-emitting filaments appears to be associated with enhanced magnetic fields. We also rediscuss the reddening of IC 10 and its implications for its distance. In the case of NGC 6822 we found only very weak evidence for nonthermal emission, except perhaps for some regions associated with local gas compression. We detect in both galaxies small spots of polarized emission, indicative of regular fields ( $\simeq$3 $\mu$G), at least partly associated with local compressional phenomena.

Key words: polarization -- galaxies: irregular -- galaxies: magnetic fields, galaxies: individual: IC10, NGC 6822 -- radio continuum: galaxies

Offprint request: K. Chyzy, chris@oa.uj.edu.pl

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