Published in "The Interstellar Medium in Galaxies",
eds. Harley A. Thronson, Jr. and J. Michale Shull, 1990.
For a postscript version of the article, click
here.
Introduction. The search for magnetic fields in the Galaxy was started already at the turn of this century, soon after polarization characteristics of the Felspar crystal were discovered. Additional impetus came from the development of polaroid foil which made this observing technique available even to amateur astronomers. The first substantiated discovery is due to Meyer (1920) who measured the polarization of the Hubble's variable nebula NGC2261. This was an observation of the polarization of a galactic source but it showed that magnetic fields exist and play an important role in the universe. At the same time methods of measuring of the solar magnetic field were being developed. Theoretical arguments for the existence of magnetic fields in galaxies were based on the need of confinement of cosmic particles (e.g. Fermi, 1949). The discovery of a magnetic field in an external galaxy is due to Öhman (1942) who used first a Felspar polarimeter and later a Wollaston prism to observe the polarized emission in Andromeda nebula (M31).
The progress in the measurement of magnetic fields in galaxies using optical polarization methods was slow, since the observations were very difficult. It was the discovery of the radio polarization of the synchrotron emission which added a new and important technique for studying magnetic fields. Also the Zeeman effect in HI clouds (and more recently in OH, H2O, CCS sources) added new data on magnetic fields in dense molecular clouds in the Galaxy. The progress in the past ten years was basic in giving us an insight into the morphology of the magnetic fields in galaxies.
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