Gießübel, René

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Kölner UniversitätsPublikationsServer

AbstractWith a distance of only 750 kpc (2.5 million light years), the Andromeda galaxy(M31) is the nearest grand spiral galaxy. Its proximity allows us to reach spatialresolutions in the galaxy of less than a kiloparsec with single dish observations,which in other galaxies can only be achieved with interferometers.It is thus no surprise that M31 was one of the first external spiral galaxies, fromwhich polarized radio emission was detected. Most of the radio continuum emis-sion is synchrotron emission, which originates from cosmic ray electrons thatspiral around the interstellar magnetic field lines. Already early analysis of thepolarized emission has shown, that the turbulent and large-scale magnetic field isconcentrated in a ring-like structure at 10 kpc radius. This “ring” is a superposi-tion of several spiral arms with small pitch angles, seen under a high inclination.Faraday rotation measures (RM) showed that the regular field of M31 is coherent,i.e. it preserves its direction around 360◦ in azimuth and across several kiloparsecsin radius.The presence of a regular field over such long distances can be explained bygalactic dynamo theory. However, the structure of the M31’s large-scale magneticfield is unusually simple. An almost purely axisymmetric field is not known fromany other spiral galaxy.The aim of this work is to provide new clues to the understanding of M31’s mag-netic field structure, by utilizing new methods for data analysis, better resolutionsand higher sensitivities than ever before, and by advancing into new wavelengthregimes: Observations of the highest and lowest frequencies of any M31 radiocontinuum polarization survey are presented. The λ3.6 cm survey is still ongoing,but a preliminary map is shown.At λ92 cm the first detection of polarization in a nearby galaxy at λ > 22 cm(factor 4 increase in wavelength) is shown. The results are consistent with currentdepolarization models.A catalogue of the detected polarized background sources at 92 cm is given, andthe analysis of these sources reveals that their internal structure is important.Classification and selection is needed for future observations with LOFAR inorder to use a grid of polarized background sources to probe magnetic fields inthe foreground.The new RM Synthesis method (Brentjens & de Bruyn 2005) was employed forthe analysis. It was developed with modern telescopes in mind, which provide alarge bandwidth divided into hundreds of channels. I also show, how this methodcan also be applied to old existing data.The extended 6 cm survey represents the deepest polarization map of an external1galaxy ever observed with the Effelsberg telescope. Weak polarized emission isdetected far away from the disk, much further than what is known from previoussurveys.With the high resolution multi-frequency polarization study of M31’s central re-gion, it was possible to uncover the full three-dimensional structure of the inner-most 0.7 kpc. The results are in good agreement with the model of a quadrupoleaxisymmetric spiral field by Braun et al. (2010). However, the direction of themagnetic field is opposite to the known field direction in the outer ring. Theinner and outer fields are thus independent from each other, which was alreadypredicted by Ruzmaikin & Shukurov (1981).Full page colour representations of the Effelsberg maps and the full WSRT fieldas well as Faraday spectra of all polarized background sources detected at 92 cmcan be found in the Appendix.2

Polarimetry of M31 from Megahertz to Gigahertz Frequencies

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