double galaxyTwo galaxies observed in close proximity. Physical doubles are spatially close pairs assumed to be in orbit about a common center of mass. They are known as a dumbbell galaxy when two elliptical galaxies of similar mass orbit each other in a common envelope. Optical doubles are widely separated pairs that appear close only because of a chance alignment along the line of sight. Although orbital motions cannot be directly observed, the relative radial velocities of galaxies in a gravitationally bound pair can be used to estimate their total mass, and hence their mass-to-luminosity ratio.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006