Maik Wolleben

The Magnetic Field of the Large Magellanic Cloud Revealed Through Faraday Rotation

We have measured the Faraday rotation toward a large sample of polarized radio sources behind the Large Magellanic Cloud (LMC) to determine the structure of this galaxys magnetic field. The magnetic field of the LMC consists of a coherent axisymmetric spiral of field strength ∼1 microgauss. Strong fluctuations in the magnetic field are also seen on small (<0.5 parsec) and large (∼100 parsecs) scales. The large bursts of recent star formation and supernova activity in the LMC argue against standard dynamo theory, adding to the growing evidence for rapid field amplification in galaxies.

Faraday screens associated with local molecular clouds

Polarization observations at λ 21 cm and λ 18 cm towards the local Taurus molecular cloud complex were made with the Effelsberg 100-m telescope. Highly structured, frequency-dependent polarized emission features were detected. We discuss polarization minima with excessive rotation measures located at the boundaries of molecular clouds. These minima get less pronounced at the higher frequencies. The multi-frequency polarization data have been successfully modeled by considering magneto-ionic Faraday screens at the surface of the molecular clouds. Faraday rotated background emission adds to foreground emission towards these screens in a different way than in its surroundings. The physical size of the Faraday screens is of the order of 2 pc for 140 pc distance to the Taurus clouds. Intrinsic rotation measures between about −18 rad m −2 to −30 rad m −2 are required to model the observations. Depolarization of the background emission is quite small (compatible with zero), indicating a regular magnetic field structure with little turbulence within the Faraday screens. With observational constraints for the thermal electron density from Hα observations of less than 0. 8c m −3 we conclude that the regular magnetic field strength along the line of sight exceeds 20 µG, to account for the observed rotation measure. We discuss some possibilities for the origin of such strong and well ordered magnetic fields. The modeling also predicts a large-scale, regularly polarized emission in the foreground of the Taurus clouds which is of the order of 0.18 K at λ 21 cm. This in turn constrains the observed synchrotron emission in total intensity within 140 pc of the Taurus clouds. A lower limit of about 0.24 K, or about 1. 7K /kpc, is required for a perfectly ordered magnetic field with intrinsic (∼75%) percentage polarization. Since this is rather unlikely to be the case, the fraction of foreground synchrotron emission is even larger. This amount of synchrotron emission is clearly excessive when compared to previous estimates of the local synchrotron emissivity.

H I Absorption of Polarized Emission: A New Technique for Determining Kinematic Distances to Galactic Supernova Remnants

We present a new method of determining the systemic velocity of Galactic supernova remnants (SNRs) based on H I absorption of their linearly polarized radio continuum emission. Conventional H I observations of total power emission are limited by H I emission and self-absorption along the line of sight, but since H I emission is unpolarized, the only limits on measurements of absorption of the polarized emission are noise and velocity resolution. This leads to lower uncertainties and makes it possible to obtain absorption profiles for virtually all Galactic SNRs with very precise H I column densities. To demonstrate the new technique, we have obtained H I absorption profiles from Tychos supernova remnant (G120.1+1.4). Absorption profiles of the polarized emission are very similar to those of the total power emission. Optical depths from the polarization profiles are slightly larger because of small-scale emission features. We also observed polarization absorption profiles of the Boomerang pulsar wind nebula (part of G106.3+2.7) and the plerionic SNR DA 495 (G65.7+1.2), remnants that are so faint that absorption profiles cannot be obtained in total power.

Polarization surveys of the galaxy

We report on sensitive 21cm and 11cm polarization surveys of the Galactic plane carried out with the Effelsberg 100-m telescope at arcmin angular resolution and some related work. Highly structured polarized emission is seen along the Galactic plane as well as up to very high Galactic latitudes. These observations reflect Faraday effects in the interstellar medium. Polarized foreground and background components along the line of sight, modified by Faraday rotation and depolarization, add in a complex way. The amplitudes of polarized emission features are highly frequency dependent. Small-scale components decrease in amplitude rapidly with increasing frequency. We stress the need for sensitive absolutely calibrated polarization data. These are essential for baseline setting and a correct interpretation of small-scale structures. Absolutely calibrated data are also needed to estimate the high-frequency polarized background. A recent study of polarized emission observed across the local Taurus-Auriga molecular cloud complexes indicates excessive synchrotron emission within a few hundred parsecs. These results suggest that possibly a large fraction of the Galactic high latitude total intensity and polarized emission is of local origin.