Search

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.