Magnetic Field and Cosmic Rays

Two important "unseen components" of the galaxy are magnetic fields and cosmic rays.
Magnetic Fields

The disk of the galaxy is permeated by a magnetic field. This field is weak, being only about 1/50,000 of the strength of the Earth's magnetic field at the surface, but it influences the motion of charged particles in the galaxy.

Polarization of Light
We can infer that our galaxy and other galaxies have magnetic fields through observation of light polarized by scattering from dust. The top right Limber Observatory image shows the spiral galaxy NGC 4565 in the Coma Berenices cluster. Superposed on the image of the galaxy are short black lines that indicate the polarization of light coming from the galaxy. The length of a line is proportional to the amount of polarization and the orientation of the line gives the polarization plane for light coming from that part of the galaxy. Part of this polarization is associated with magnetic fields, because magnetic fields can align the spin of elongated dust particles so that light scattered from them acquires a preferred direction of polarization. Thus, such polararization data allow one to study magnetic fields in galaxies. In a similar way, the magnetic field in our own galaxy has been investigated by studying the polarization of light transmitted through the dust clouds lying in the disk.
Origin of Galactic Magnetic Fields
The origin of the galactic magnetic field is not well understood. At one time it was thought that the galaxy was formed with a magnetic field already in place, but prevailing current opinion is that the magnetic field is generated by a dynamo similar to that responsible for planetary and solar magnetic fields. In this picture, currents in the plasma contained in the disk of the galaxy are modified by the rotation of the galaxy (Coriolis forces and differential rotation of the disk) to produce a dynamo effect.
Cosmic Rays
The galaxy also contains the high-energy charged particles that we call cosmic rays. About 90 percent of cosmic rays are hydrogen nuclei, with the remainder being heavy nuclei and electrons. The more energetic cosmic rays have energies that are higher than that produced in any particle accelerator on Earth. One important consequence of the galactic magnetic field is that it can bend the path of and even trap cosmic rays. Because the magnetic field bends their path, we cannot tell from which direction cosmic rays originally came. Thus, we are not certain about the source of cosmic rays, but it is thought that most result from the acceleration of particles in supernova remnants. At least some of the highest energy cosmic rays may come from beyond our own galaxy, perhaps from active galactic nuclei (which are described in Chapter 25).