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Date and Time of the Query: 2019-06-25 T04:18:35 PDT
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For refcode 1991A&A...241...47B:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1991A&A...241...47B A multifrequency radio continuum survey of M 33 III. The magnetic field U. R. Bnczilowski and R. Beck Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-5300 Bonn 1, Federal Republic of Germany Received February 2, accepted June 23, 1990 Abstract. Linearly polarized emission of M33 measured with the Effelsberg 100-m telescope between 21.1 cm and 6.3 cm is concentrated to the northern half of the galaxy. The low degree of uniformity in the southern half of M 33 is possibly due to the higher star formation rate increasing the turbulence velocities of interstellar clouds and tangling the field lines. There is almost no polarization detectable at the positions of optically identified supernova remnant (SNR) candidates. They may have disturbed the interstellar magnetic field on scale sizes of ~100 pc, smaller than our beam and hence reduce the observable polarized flux. The depolarization increases only slowly from 6.3 cm to 21.1 cm which indicates a dominating random component of the magnetic field which is unresolved on scales sizes < 500 pc plus a resolved uniform component on a scale size > 3.4 kpc. The average degree of linear polarization of the synchrotron emission P_nth_ = 11 +/- 5% at 6.3 cm corresponds to an average ratio between the strength of the uniform and random field of B_u_/B_r_ = 0.4 + 0.1 within the resolution element of 510 x 880 pc. M 33 has the lowest field uniformity among 7 nearby galaxies for which comparable estimates are available. The foreground Faraday rotation measure of M 33 is RM_fg_ = 55 +/- 7 rad m^-2^. The internal rotation measure RM_0_= 15 + 5 rad m^-^ is only half of the typical value for spirals. A method to determine the strength of the uniform magnetic field from the internal rotation measure is described. Using either the internal rotation measures or the commonly used minimum energy requirement we find an average total magnetic field strength of B_t_ = 4 +/- 1 {mu}G. This represents one of the weakest fields in the sample of nearby spiral galaxies studied so far. The corresponding low Alfven speed of ~50 km s^-1^ cannot explain the propagation of cosmic ray electrons ~6 kpc off their sources, a strong increase of the Alfven speed with distance from the centre is suggested. The magnetic field lines generally follow the spiral arms with increasing pitch angle in the outer regions of the galaxy. The field lines are not closed within the measurable disk of r ~6 kpc. There are no indications for field reversals either between different radii on scales larger than 3 kpc or with respect to the galactic plane. The magnetic field structure and the rotation measure distribution are consistent with a simple model bisymmetric within and probably symmetric to the galactic plane. In terms of the dynamo theory M 33 seems to show an even m = 1(S1) mode. Finally the possible relation between the global structures of spiral arms and magnetic field configurations are discussed for the data available now for the sample of spiral galaxies. Key words: spiral galaxies: M 33 radio continuum polarization - Faraday effects - interstellar magnetic field
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