The Nuclei of Nucleated Dwarf Elliptical Galaxies - Are they Globular Clusters?

Abstract

It came as a great surprise that many dwarf elliptical galaxies of very low surface brightness in the Virgo Cluster have conspicuous bright star-like nuclei (Reaves 1983, Binggeli, Sandage and Tammann 1985). These nuclei are at least a factor of 10 more luminous than the brightest globular clusters in the Local Group and comparable only to the very brightest globulars surrounding M87. They contain a considerable fraction (1 to 20%) of the total light of the parent galaxy (Binggeli, priv. commun.). Their physical nature and origin are a matter of debate (Zinnecker et al. 1985, van den Bergh 1985, Norman 1986, Zinnecker 1986) but optical spectroscopy for 3 objects indicates a stellar composition with a range similar to globular clusters (Bothun et al. 1985). It has been suggested that a central nucleus is formed when off-center bound star clusters migrate to the center as a consequence of dynamical friction (Norman 1986). Support for such a scenario comes from CCD observations of IC 3475 which reveal numerous knots near the center of this dwarf irregular galaxy (Vigroux et al. 1986). These knots have the same color as the parent galaxy and are interpreted as intermediate age star clusters.