4. Illustration of individual cases
The large uncertainties involved call for a statistical treatment of the subject, which is carried out below. First we show, in Fig. 2, a few cases with significant nuclear off-center displacements, to illustrate the phenomenon as such and to show how the procedure to determine a mean nuclear offset in the isophotal range 24-25 B arcsec-2 works. All six dwarfs have an average nuclear offset of 1" or more (for the exact values see Table 1), which is larger than what could be explained by systematic and random errors alone (as explained below), i.e. the displacements are very likely real. In fact, in all cases but one (VCC1386) the displacements are easily recognized by eye.
Notes on individual galaxies (see Fig. 2):
VCC389: This is a bright dwarf S0 galaxy, classified as such because of a distinct and clearly visible two-component structure: there is an inner, lens-like feature of high surface brightness, with a fairly sharp edge, on top of a more extended low-surface brightness part (for the morphology of dS0s, see Sandage & Binggeli (1984) and BC91). If we look how the nuclear distance to the center correlates with the surface brightness level at which the center was determined from fitting an ellipse, we see that the nucleus is quite central (only insignificantly displaced) in the inner part, but then jumps to 1" beyond 24 B arcsec-2. Obviously, the nucleus is central with respect to the inner "lens", while the whole lens seems to be displaced with respect to the rest of the galaxy.
VCC870: Again a bright dS0,N with a lens-like structure in the inner part. In contrast to VCC389, however, the nucleus is clearly displaced already within the lens and it remains so, albeit with a strange bump in the relation, with respect to the outer part. Such cases, where the nucleus is shifted against the whole galaxy, including the inner part, seem to be rare.
VCC1386: This is an ordinary, bright dE,N. A nuclear displacement sets in at 24 B arcsec-2 and rapidly increases towards fainter surface brightness. Nevertheless, the effect is very hard to see by eye.
VCC1563: A fainter dE,N with a "well-behaved" and easily visible off-center nucleus.
VCC1796: This dwarf is again slightly peculiar as dE, having explicitly been classified as "dE5:, N:" (the colon means uncertainty). Yet the nuclear offset is obvious.
© European Southern Observatory (ESO) 2000
Online publication: July 7, 2000