EDP Sciences
Free access
Volume 444, Number 3, December IV 2005
Page(s) 739 - 765
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20053556

A&A 444, 739-765 (2005)
DOI: 10.1051/0004-6361:20053556

Magnetic fields in barred galaxies

IV. NGC 1097 and NGC 1365
R. Beck1, A. Fletcher1, 2, A. Shukurov2, A. Snodin2, D. D. Sokoloff3, M. Ehle4, D. Moss5 and V. Shoutenkov6

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: rbeck@mpifr-bonn.mpg.de
2  School of Mathematics and Statistics, University of Newcastle, Newcastle upon Tyne NE1 7RU, UK
3  Department of Physics, Moscow State University, 119992 Moscow, Russia
4  XMM-Newton Science Operations Centre, European Space Astronomy Centre (ESAC), European Space Agency, PO Box 50727, 28080 Madrid, Spain
5  School of Mathematics, University of Manchester, Manchester M13 9PL, UK
6  Pushchino Radioastronomy Observatory, Astro Space Center, 142292 Pushchino, Russia

(Received 2 June 2005 / Accepted 2 August 2005)

We present $\lambda3.5$ cm and $\lambda6.2$ cm radio continuum maps in total and polarized intensity of the barred galaxies NGC 1097 (at 2´´-15´´ resolution) and NGC 1365 (at 9´´-25´´ resolution). A previously unknown radio galaxy southwest of NGC 1097 is reported. Apart from a smooth faint envelope and a bright central region, both galaxies exhibit radio ridges roughly overlapping with the massive dust lanes in the bar region. The contrast in total intensity across the radio ridges is compatible with compression and shear of an isotropic random magnetic field, where the gas density compression ratio is approximately equal to 4 and the cosmic ray density is constant across the ridges. The contrast in polarized intensity is significantly smaller than that expected from compression and shearing of the regular magnetic field; this could be the result of decoupling of the regular field from the dense molecular clouds. The regular field in the ridge is probably strong enough to reduce significantly shear in the diffuse gas (to which it is coupled) and hence to reduce magnetic field amplification by shearing. This contributes to the misalignment of the observed field orientation with respect to the velocity vectors of the dense gas. Our observations, for the first time, indicate that magnetic forces can control the flow of the diffuse interstellar gas at kiloparsec scales. The total radio intensity reaches its maximum in the circumnuclear starburst regions, where the equipartition field strength is about $60\,\mu{\rm G}$, amongst the strongest fields detected in spiral galaxies so far. The regular field in the inner region has a spiral shape with large pitch angle, indicating the action of a dynamo. Magnetic stress leads to mass inflow towards the centre, sufficient to feed the active nucleus in NGC 1097. - We detected diffuse X-ray emission, possibly forming a halo of hot gas around NGC 1097.

Key words: galaxies: magnetic fields -- galaxies: individual: NGC 1097, NGC 1365 -- galaxies: spiral -- galaxies: structure -- ISM: magnetic fields -- galaxies: nuclei

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© ESO 2005