Search

We present a radio continuum survey of M31 at λ6.2 cm with high sensitivity in total power and polarization, observed with the 100-m Effelsberg dish with an angular resolution of 2'.4. (1) Combination with the VLA + Effelsberg map at λ20.5 cm in total power yielded a spectral index map at 3' resolution. Both the spectrum of the integrated emission and the spectral index distribution across M 31 indicate a nonthermal spectral index α n = 1.0 ′ 0.2. We derived maps of thermal and nonthermal emission at λ6.2 cm. Radial profiles of the various emission components north and south of the minor axis revealed that the stronger total emission in the northern part of M 31 is entirely due to stronger thermal emission, whereas the profiles of nonthermal and polarized emission are nearly identical on either side of the minor axis. This suggests that recent star formation does not lead to a local increase of the number of relativistic electrons and/or magnetic field strength. (2) We discuss several properties of the polarized emission from M 31 and their implications for the magnetic field. At λ6.2 cm the polarized intensity systematically varies along the bright "ring" of emission which shows that the regular magnetic field, B r e g , is nearly aligned with the spiral arms forming this "ring". The variation of the rotation measures between λ 11.1 cm and λ6.2 cm, RM(11, 6), across the galaxy confirms this alignment. The nonthermal degree of polarization reaches values >50% near the polarization maxima, implying that the magnetic field in M31 is exceptionally regular. (3) We derived the distribution of the nonthermal depolarization between λ11. I cm and λ6.2 cm, DP n (11.6), which is a measure of Faraday depolarization. Gradients in RM( 11.6) may be an important cause of Faraday depolarization in M 31. The lack of anticorrelation between the thermal emission, which comes mainly from dense H u regions with small filling factors, and RM(11, 6) and DP n (11, 6) indicates that rotation measures and Faraday depolarization originate in the extended diffuse ionized gas.

We present a survey of the total and linearly polarized radio continuum emission of the Andromeda galaxy at 20 cm wavelength with the VLA-D array. 7 elds were observed with 45 00 angular resolution. The miss- ing large-scale total emission observed with the Eelsberg telescope was inserted. This survey is the most sensitive radio continuum survey of M 31 so far and the rst one in linear polarization with the VLA. The total emission of M 31 comes from extended re- gions and point sources. Both components are concen- trated in the bright \ring" of maximum star formation at about 10 kpc radius. Nearly half of the extended emis- sion emerges from numerous lamentary features which are typically a few arcminutes long (' 600 pc). In the eld of M 31, 36 linearly polarized background sources were detected. The smallest degrees of polarization occur on the cen- tre of the \ring" where the total emission is highest, hence on the spiral arms as delineated by H emission and OB associations. The distribution of polarization angles re- veals coherent features of typically 1 kpc in extent, some of which show a relation to an OB association.

MAGNETIC fields pervade spiral galaxies1—at least all those searched so far; these fields have a substantial energy density, and hence could play an important role in galaxy evolution. Dynamo theory has been used for many years to explain the presence and overall structure of galactic magnetic fields, through the amplification of a weak seed field2,3. Here we report the observation of two 'magnetic spiral arms' in the nearby galaxy NGC6946, lying between the optical spiral arms. This is surprising because dynamo action is thought to be related to star formation activity4, which is concentrated within or in the leading edges of the optical spiral arms. The magnetic spiral arms are about 500–1,000 parsecs wide and more than 12 kiloparsecs long, and have greater symmetry than the optical arms. This organized structure probably reflects the signature of some global mechanism relating to magnetic field generation, but no current theory—in particular dynamo theory in its present form—is able to explain this phenomenon.