The Origins of Disk Heating

By making spectral absorption-line observations of disk galaxies at intermediate inclinations, we have been able to determine the amplitude of their constituent stars' random motions in three dimensions. This full measure of the shape of the velocity ellipsoid is a useful diagnostic for determining the "heating" mechanism responsible for creating the random motions. The analysis implies that the main heating process varies with galaxy type, with minor mergers dominating the heating in early-type disk galaxies, and spiral density waves the most important mechanism in late-type systems.