Title:
Polarized radio emission of edge-on galaxies: Observational results and implications.
Authors:
DUMKE, M.; KRAUSE, M.; WIELEBINSKI, R.; KLEIN, U.
Publication Date:
1995

Abstract

We performed a small survey of the total and linearly polarized emission of six edge-on galaxies at 2.8cm. The selected galaxies show very different levels in their star forming activity, their environment (interacting or not), and many other properties. In the following we present the main results of these observations.
Magnetic field orientation: No polarized emission could be detected in NGC 5907. But the other galaxies in our sample do all show a large-scale magnetic field predominantly parallel to the galactic plane, except NGC 4631: this strongly interacting starburst galaxy shows a vertical magnetic field structure in the central area. Thus it is obvious that dominant magnetic fields perpendicular to the disks of spiral galaxies are the exceptional rather than the normal case. But it is yet not clear which circumstances lead to such a field configuration. NGC\,3628, also a starburst galaxy, shows an ordered magnetic field parallel to the disk.
Radio spectra: From a combination of our measured flux densities with already published values at other frequencies we found spectral indices {alpha} ranging from -0.67 to -0.86, consistent with the values found for normal disk galaxies. Nevertheless the spectra seem to flatten with increasing nuclear star forming activity.
Galactic halos: We measured the distribution of the total intensity perpendicular to the galactic disk. Although we detected emission above the disk in all galaxies (except NGC 7331), only two of them show an extended radio halo. While NGC 4631 exhibits a galactic wind, NGC 891 is thought to be in a "chimney-mode", where disk material is ejected into the halo through chimneys.
Depolarization: The fractional polarization is found to be always lower than the theoretical value of ~ 74% and to increase with increasing distance from the galactic plane. Furthermore the amount and rate of this increase is different in all galaxies. Since we can neglect Faraday effects at this wavelength, we modelled the degree of polarization, based on beam depolarization. We found that the correlation length of the magnetic field increases with galactic height z and increases faster in galaxies with lower star formation efficiency, supporting the idea that strong star formation leads to the transport of turbulences from the disk into the halo.

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