Since many, if not most non-active galactic nuclei are suspected to harbour a black hole (e.g. Kormendy & Richstone 1995), interest in these nuclei is no longer peripheral. Aside from their black hole "attraction", the nuclei are likely telling us something about the star formation history and the dynamics of the innermost part of galaxies. Among the most intriguing nuclei are those in dwarf elliptical (dE) galaxies (for a review on dEs see Ferguson & Binggeli 1994). Due to the low surface brightness of the underlying galaxy, the dE nuclei are not only popping out most clearly, their very formation in a low-density environment is a riddle. The most nearby dE nucleus is that of the Andromeda satellite NGC 205, which is of course known at least since Baade (1944). The universality of the dwarf nucleus phenomenon became clear with the Las Campanas survey of the Virgo cluster (Binggeli et al. 1985, preceded by Reaves 1983) where 400 nucleated dEs (out of 800 dEs in total) were identified. The morphology of "dE,N" galaxies is illustrated in the dwarf galaxy atlas of Sandage & Binggeli (1984).
The nature of the dE nuclei is still unclear, but most likely they are massive compact star clusters (like the nuclei in M33 and other low-luminosity spirals; Kormendy & McClure 1993, Phillips et al. 1996, Matthews et al. 1999) constituting separate dynamical entities, but without being totally decoupled from the rest of the underlying galaxy. Their formation is still more speculative. One plausible scenario regards the dE nuclei as the fossils of the last bursts of star formation in the evolutionary transition from dwarf irregulars (or blue compact dwarfs, BCDs) to dwarf ellipticals (Davies & Phillipps 1988; however, see, e.g., Durrell 1997 and Miller et al. 1998 for the shortcomings of the Irr to dE scenario).
In this paper we address the question of how central the dwarf nuclei are. The nuclei have of course to be fairly central to be noticed as such, but still they could be significantly off-center relative to the radially symmetric, global light distribution of the parent galaxy. Why should this be interesting? Suppose the nuclei were formed secularly by the merging of globular clusters through dynamical friction, or evolved from non-central star burst regions in dwarf irregulars, as in the Davies & Phillipps scenario mentioned above. Depending on the age of the nucleus we might expect them to be off-center to various degrees. Miller & Smith (1992) have shown that even old nuclei would not patiently sit in the center of a galaxy. Such a state would be dynamically unstable; a nucleus is bound to oscillate in the potential of a galaxy. Taga & Iye (1998), performing N-body simulations of a rotating spherical galaxy with a central black hole, have found that the amplitude of oscillation depends on the mass of the black hole (or the nucleus) relative to the mass of the underlying galaxy. The simulations suggest that the amplitude will also depend on the central mass density of the underlying galaxy. These things have still to be worked out in detail by further numerical experiments.
In any case, there are enough hints from theory that off-center nuclei should be widespread, and that a quantification of the nuclear offsets might help us to put constraints on the formation, mass, and dynamics of the nuclei and the inner region of galaxies. Several off-center nuclei have been discovered by the HST survey of the centers of giant elliptical (E) galaxies (Lauer et al. 1995). However, part of these nuclear offsets in Es could be artifacts due to dust obscuration, or be caused by unresolved off-center sources. Spectacular cases of such off-center sources are the apparent double nuclei in the bulge of M31 and the compact elliptical NGC 4486B (Lauer et al. 1993, 1996), which are most plausibly modelled by an eccentric nuclear disk (Tremaine 1995).
The present paper is a first attempt to find off-center nuclei in dwarf elliptical galaxies, and to quantify the nuclear off-center distances, based on a homogeneous sample of 78 nucleated Virgo cluster dEs.
The rest of the paper is organized as follows. In Sect. 2 we define our sample of nucleated dwarfs, and in Sect. 3 we describe the procedures adopted to measure off-center positions. Some individual cases of off-center nuclei, illustrating the phenomenon and the measuring procedure, are shown and discussed in Sect. 4. To get reliable statistics for off-center nuclei we had to account for systematic and random errors of the measuring procedure, which we did by running Monte Carlo simulations with artificial galaxies, as described in Sect. 5. The statistical results are then presented in Sect. 6, showing the general trends for the whole sample. Our conclusions are given in Sect. 7.
In an appendix we address the relation between nuclear brightness and ellipticity. This feature is related to our main topic of off-center nuclei insofar as it may also have some bearing on the mass of the nuclei and the possibility that they harbour a black hole.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000