Magnetic Field Generation during Galaxy Formation

The detailed study of the interstellar medium in the Milky Way has given rise to a rich history of papers on magnetic field generation in the disks of spiral galaxies (eg Parker 1971). The theoretical models for dynamo action can now be tested because recent advances in radio polarimetry have allowed detailed polarization maps of nearby spiral galaxies to be completed. The observations suggest that galaxies have both symmetric and bisymmetric spiral magnetic field patterns (eg. Sofue et al 1986). Little attention has been given to the nature of magnetic fields in galactic halos however. This arises in part because direct observations of such fields requires that a good subtraction of the background faraday rotation of our own galactic disk be achieved. Recently, Hummel et al (1988) have directly mapped the structure of a halo magnetic field around the edge-on spiral galaxy NGC 4631. This field is as strong as 3.5 μg out to 6 kpc above the plane and is suggestive of a dynamically important halo field. Welter, Kronberg, and Perry (1984) have shown that regions in galaxies as large as 50 kpc harbour magnetic field as strong as 1–10 μQ. There is a significant trend of rotation measure with redshift in that the systems at larger redshift have larger rotation measures. This may indicate that young and perhaps protogalactic halos could constitute some of the intervenors.