Off-center Nuclei in Dwarf Elliptical Galaxies

Abstract

We have searched for off-center nuclei in 78 "nucleated" dwarf elliptical (dE,N) galaxies, drawing on digitized photographic images from a previous study of Virgo cluster dwarfs. The search is based on a simple algorithm which compares the center coordinates of a series of outer elliptical isophotes with the position of the galaxy's nucleus. Monte Carlo simulations of the measuring procedure are used to assess random and systematic errors. Roughly 20% of all dwarf nuclei in the sample (neglecting uncertain cases) are found to be significantly offcentered. The typical displacement is 1″, or 100 pc (assuming a Virgo cluster distance of 20 Mpc), corresponding to 0.5 to 1 effective radii of the dwarf galaxy. There is a tendency of the nuclear off-center displacement to increase with decreasing surface brightness of the underlying galaxy. A similar trend was found with normal elliptical galaxies before. If real, the effect could mean that a nucleus can oscillate about the galaxy center with larger amplitude in a shallower (less cuspy) gravitational potential. In an appendix we present evidence for the existence of a strong, unambiguous relation between the nuclear magnitude and the ellipticity of dE,N galaxies. If a nucleus is comprising 4% or more of the total light of the underlying galaxy, that galaxy is nearly always round, i.e. ellipticity less than 0.15 (dE0, dE1). This effect was predicted qualitatively long ago as the result of box orbit disruption caused by a central massive compact object (black hole). Show more