Observations of Magnetic Fields in Galaxies

Abstract

Magnetic fields play an important role in galaxies. Interstellar magnetic fields are strongest in massive spiral arms of galaxies (20–30 μG) and in nuclear starburst regions (up to 100 μG). Processes related to star formation tangle the field lines, so that little polarization is observed in star‐forming regions. The magnetic energy density in the inner disk of galaxies is larger than the thermal energy density, comparable to that of turbulent gas motions, and is dominant in the outer disk. Large‐scale spiral patterns of the regular field are observed in grand‐design, flocculent and even some irregular galaxies. In grand‐design galaxies the regular fields are aligned parallel to the optical spiral arms, with the strongest regular fields (highest polarization) in interarm regions, sometimes forming magnetic spiral arms between the optical arms. Polarized emission at the edges of some galaxies is a tracer of field compression due to interactions between galaxies or to ram pressure by the intergalactic mediu...