Magnetic fields as an alternative explanation for the rotation curves of spiral galaxies

Abstract

The flat rotation curves of spiral galaxies are usually regarded as the most convincing evidence for dark matter. The assumption that gravity alone is responsible for the motion of gas beyond the visible disks of galaxies leads directly to the conclusion that there must be perhaps 10 times as much dark matter as visible matter. Other forces besides gravity are usually neglected, as order-of-magnitude arguments seem to suggest they cannot be important. The existence of dark matter is, however, so important an issue that we believe it is wise to consider other possibilities. Here we argue that an azimuthal magnetic field can carry slightly ionized gas with the general galactic rotation, rendering dark matter unnecessary (a related idea was first proposed by Nelson1). For the illustrative case of M31, a magnetic field of 6 μG is required, and the synchrotron emission of relativistic electrons in this field is compatible with the observations.