Moss: Magnetic fields in spiral galaxies

The first hints that our galaxy, the Milky Way, might host a large-scale magnetic field came in the late 1940s, from studies of the polarization of starlight. However, optical polarization is a relatively blunt tool and it was only in the late 1950s that radio synchroton observations began to indicate the presence of large-scale fields (e.g. Gardner and Davies 1966). By the early 1980s observations of linearly polarized radio emissions (PI) were giving unambiguous evidence for the presence of large-scale magnetic fields of microgauss strength in the Milky Way and nearby spiral galaxies (e.g. Beck et al. 1996). Note that small-scale (“random”, “turbulent”) fields are also universally present, detectable by their unpolarized synchrotron emission, and they are at least as strong (usually stronger) than the ordered component. Note also that polarization measurements do not unambiguously determine field direction – there is a 180° uncertainty. To resolve this ambiguity, multiwavelength Faraday rotation measures (RM) are necessary, a much more demanding requirement. At present, there is detailed knowledge of field strengths and structure in many nearby spiral galaxies and Local Group irregulars – see figures 1, 2 and 3 for some examples. Total field strengths vary from a few microgauss in radio faint galaxies such as M31 and M33 (figure 2), to 50–100 μG in starburst galaxies such as NGC 4038/9 and in nuclear starbursts such as seen in NGC 1097. It appears that ordered magnetic fields are universally found in well-observed spiral galaxies, and generally trace (maybe rather patchily) trailing spiral patterns, with pitch angles typically in the range 10–40°. Usually the degree of polarization within optical spiral arms is only a few percent, and regular magnetic fields are found predominantly in between optical arms, with small-scale fields dominating within the arms, e.g. figure 3. The pitch angles usually are not identical with those of the optical arms. In barred galaxies, field vectors tend to be aligned with the strong streaming velocities.