7. Summary and conclusions
With very simple means, by fitting ellipses to the outer galaxy part and measuring the distance of the nucleus to the ellipse centers, we have searched for the existence of off-center nuclei in a sample of 78 nucleated dwarf elliptical galaxies of the Virgo cluster. Simulating the measuring procedure with artificial galaxies showed that nuclear displacements of more than 0.8" can safely be regarded as real. Taking this into account, we found that roughly 20% of all nucleated dEs have off-center nuclei.
In search for a relation between the nuclear displacement and any kind of structural and environmental parameter of the underlying galaxy, we found a possible tendency of the nuclear displacement (or its spread) to increase with decreasing surface brightness of the underlying galaxy. A similar trend (but even less significant, given the small number of objects involved) appeared in Lauer et al.'s (1995) study of the centers of normal elliptical galaxies. If confirmed to be real, the trend could simply mean that the nuclei are less strongly bound in a shallow gravitational potential, moving more easily away from, or oscillating with larger amplitude about, the center of the underlying galaxy (see Miller & Smith 1992, and Taga & Iye 1998), whatever the excitation or driving mechanism for this motion may be.
A typical off-center distance in a Virgo dE,N is 1", or 100 pc with =20 Mpc. With respect to the size of the dwarfs, this corresponds to 0.5 to 1 effective radii. But it has to be borne in mind that dwarfs with nuclei more displaced from the galaxy center than 2" are strongly selected against in our sample, because such objects, if they really exist, would not always have been recognized and classified as "nucleated" (dE,N).
We have used a fairly large and homogeneous data set of digitized photographic images of dE,N galaxies. Future studies of the subject, based on higher-resolution images, will undoubtedly allow much more precise measurements of the nuclear displacements. However, as exemplified beautifully by the HST imaging of Miller et al. (1998), high resolution also opens up Pandora's box , in that all bright globular clusters become detected as well, rendering the distinction, or even definition, of a fairly faint "central" nucleus non-trivial. But this kind of entanglement is quite natural, as globular clusters and "central" nuclei are related phenomena and may well have a common origin. Nevertheless, even with superior imaging, a large sample ( 100) of dE,Ns will be needed to search for statistically significant trends of any nuclear property with the structure of the underlying galaxy, as has been attempted with archive data in the present paper.
Finally, the nuclear displacements, as well as the nuclear magnitudes in relation to galaxy shape (see appendix), will likely tell us something about, or at least allow us to put constraints on, the masses of the nuclei and the galaxy potentials in which they sit. But this is only possible based on quantitative models. Aside from the simulations of Miller & Smith (1992) and Taga & Iye (1998), we completely lack an adequate, realistic and quantitative modelling of off-center nuclei in galaxies.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000