Zeeman effect
The first detection of the Zeeman effect in another galaxy was made through observations of hydroxyl megamasers. The Zeeman effect is the splitting of a spectral line due to the presence of a magnetic field, and the size of the splitting is linearly proportional to the line-of-sight magnetic field strength. Zeeman splitting has been detected in five hydroxyl megamasers, and the typical strength of a detected field is of order a few milligauss, similar to the field strengths measured in galactic hydroxyl masers.