Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991a%26a...241...47b&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 241, no. 1, Jan. 1991, p. 47-56.
Astronomy and Astrophysics
Astronomy
28
Interstellar Magnetic Fields, Radio Astronomy, Sky Surveys (Astronomy), Spiral Galaxies, Astronomical Models, Linear Polarization, Magnetic Field Configurations, Synchrotron Radiation, Spiral Galaxies: M33, Radio Continuum, Polarization, Faraday Effects, Interstellar Magnetic Field
Scientific paper
Linearly polarized emission of M33 measured with the Effelsberg 1 00-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 M33 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 Pnth = 11±5% at 6.3 cm corresponds to an average ratio between the strength of the uniform and random field of Bu/Br =0.4±0.1 within the resolution element of 510 × 880 pc. M33 has the lowest field uniformity among 7 nearby galaxies for which comparable estimates are available.
The foreground Faraday rotation measure of M33 is RMfg = -55±7 rad m-2. The internal rotation measure RM0 = 15±5 rad m-2 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 Bt = 4±1 μG. This represents one of the weakest fields in the sample of nearby spiral galaxies studied so far. The corresponding low Alfvén speed of 50 km s-1 cannot explain the propagation of cosmic ray electrons ≃6 kpc off their sources, a strong increase of the Alfvén 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 M33 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.
Beck Rainer
Buczilowski U. R.
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