The Role of Magnetic Fields in Spiral Galaxies

Interstellar magnetic fields are strong: up to 25μG in spiralarms and 40μG in nuclear regions.In the spiral galaxy NGC 6946 the average magnetic energy densityexceeds that of the thermal gas. Magnetic fields control the evolution of denseclouds and possibly the global star formation efficiency in galaxies.Gas flows and shocks in spiral arms and bars are modified by magneticfields. Magnetic forces instar-forming circumnuclear regions are able to drive mass inflow towardsthe active nucleus. Magnetic fields are essential for the propagationof cosmic rays and the formation of galactic winds and halos.