The counter-streaming instability in dwarf ellipticals with off-center nuclei

Abstract

n many nucleated dwarf elliptical galaxies (dE,N's), the nucleus is offset by a significant fraction of the scale radius with respect to the center of the outer isophotes. Using a high-resolution N-body simulation, we demonstrate that the nucleus can be driven off-center by the m=1 counterstreaming instability, which is strong in flattened stellar systems with zero rotation. The model develops a nuclear offset on the order of 30% of the exponential scale length. We compare our numerical results with the photometry and kinematics of FCC 046, a Fornax Cluster dE,N with a nucleus offset by 1.2" we find good agreement between the model and FCC 046. We also discuss mechanisms that may cause counterrotation in dE,N's and conclude that the destruction of box orbits in an initially triaxial galaxy is the most promising.