ORIGIN OF KINEMATIC SUBSYSTEMS IN ELLIPTIC GALAXIES

ELLIPTICAL galaxies were once thought to be smooth, featureless stellar systems with little or no substructure, but increasingly sophisticated observations are challenging this point of view. Some ellipticals contain small central disks which may be counter-rotating or otherwise kinematically decoupled from the rest of the galaxy 1-6. It seems unlikely that these disks could form during the monolithic collapse of a slowly rotating proto-galaxy, as the most plausible outcome of such evolution is a system with a simple rotation pattern. Like other kinds of fine structure in elliptical galaxies 7, these subsystems have been widely interpreted as evidence for multiple formation events or episodes. Here we demonstrate the formation of a counter-rotating central gas disk in a merger of two gas-rich disk galaxies of equal mass. Such a structure may well account for the unusual gas kinematics found in the merger remnant NGC7252 (refs 8, 9). Continued star formation in such gaseous disks may produce central components with decoupled kinematics, resembling the cores of some elliptical galaxies.