Galactic polarization surveys

1 Abstract Following the detection of polarized diffuse Galactic emission in 1962 a number of surveys were undertaken at low frequencies in the following years resulting in important insights on the local magnetic field, polarization of the giant radio loops and other Galactic structures, as well as on the properties of the diffuse magnetized interstellar medium. This field of research experienced a revival in the eighties and nineties by a number of high resolution observations at low and high frequencies, which showed a large variety of polarization structures having no corresponding signature in the total intensity images. 'Canals' and 'Faraday screens' were reported, which clearly indicate that Faraday rotation in the magneto-ionic medium may largely vary on small scales. These findings called for a systematic approach and a number of new unbiased polarization surveys were started. Also new attempts for absolute calibration are under way, which is a critical issue when interpreting polarization structures. This paper reviews polarization survey projects and also summarizes recent results and interpretations of this rather active field of research. Galactic radio emission is made up from individual sources like supernova remnants (SNRs) or HII-regions, which are in their majority distributed along the Galac-tic plane and also from diffuse emission originating in the interstellar medium and extending towards high Galactic latitudes. Diffuse Galactic emission has a large volume filling factor and consists of synchrotron emission and the emission from ionized low density gas. Galactic radio sources are highly concentrated in the thin disk of the Galaxy and studies to derive their physical properties in most cases require high angular resolution observations for a wide range of radio frequencies. Polarization observations of synchrotron emitting sources like SNRs reveal information about the regularity of the magnetic field, its orientation within the object and also of the ambient interstellar field. Polarized radio emission suffers from Fara-day rotation within the emitting volume and along the line-of-sight, which again requires multi-frequency observations to solve for that. All these effects are valid for diffuse emission as well, where the situation may be even more complicated by multiple emission layers along the line of sight, what requires to model the radiation transport conditions in some detail. Studies of the polarized diffuse Galactic emission received high interest during the last decade as Faraday rotation effects in the interstellar medium are thought to be responsible for a zoo of unusual polarized structures, which have no …