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Astron. Astrophys. 359, 447-456 (2000)

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2. Sample

The present investigation draws on the photometric study of [FORMULA] 200 Virgo cluster early-type dwarf galaxies by Binggeli & Cameron (1991, 1993, hereafter BC91 and BC93, respectively), which itself is based on digitized photographic plates from the Las Campanas survey of the Virgo cluster (Binggeli et al. 1985, hereafter VCC = Virgo Cluster Catalog). For our purpose of measuring nuclear dwarf positions digitized (deep & high-resolution) photographic images are as well suited as modern CCD images. We have selected a subsample from the BC91 sample by the following constraints: (1) the dwarf object had to be classified as nucleated, i.e. dE,N or dS0,N, and (2) the apparent magnitude was required to be brighter than [FORMULA] = 18 mag, corresponding to an absolute blue magnitude of -13.5 by assuming a distance to the Virgo cluster of 20 Mpc. Note that the dS0 type is a slight, and for the present purposes insignificant, variant of the dE class (see Sandage & Binggeli 1984, and BC91). - This sample amounted to 109 objects. Due to bright stars in the central areas of the galaxies and other technical difficulties we lost about 30 images. Without introducing strong biases, this left us with a final, representative sample of 78 nucleated dwarfs. The sample galaxies with some of their fundamental photometric parameters (all from BC93) are listed in Table 1. The columns of Table 1 are as follows:

column (1): VCC number of the galaxy;

column (2): dwarf type classified in the system of Sandage & Binggeli (1984);

column (3): total apparent blue magnitude [FORMULA];

column (4): mean surface brightness within the effective radius [FORMULA] that contains half of the total light, [FORMULA] (in B arcsec-2).

The remaining columns (5)-(7) of Table 1 are defined in Sect. 3 further below.


[TABLE]

Table 1. Basic data and nuclear off-center displacements of 78 dE,N and dS0,N galaxies in the Virgo cluster


Constraining the sample to objects explicitly classified as "nucleated" is of course not without problems. Besides a bias against very faint nuclei that simply went undetected in the photographic survey of Binggeli et al. (1985), a nucleus had to be sufficiently central to be considered and classified as such, to begin with. A "nucleus" very far off the center is indeed a contradiction in terms. Consider, e.g., VCC1857 depicted in Fig. 1 (also reproduced, as IC3647, in Sandage & Binggeli 1984, Fig. 2, Panel 5). There is a bright star-like object on top of the galaxy, but so much off-center that it might be a foreground star or a background galaxy rather than the type of nucleus seen in the center of "dE,Ns"; hence the classification "dE,N?". This galaxy still happens to be included in our sample, albeit as "uncertain case" (see below).

[FIGURE] Fig. 1. The dwarf elliptical VCC1857 (=IC3647) classified as "dE,N?". Note the very large off-center position of the bright quasi-stellar object, rendering its nuclear status uncertain. Compare with the more ordinary (only slightly off-centered) nuclei shown in Fig. 2. The image is 1.2 arcmin on a side.

[FIGURE] Fig. 2. Six individual cases of dwarfs with off-center nuclei. For each galaxy we display a direct image on top of a plot showing the nuclear off-center displacement (in arcsecs) as function of the surface brightness level (in B arcsec-2) at which an ellipse was fitted and a center defined. The thin vertical lines are indicating the boundaries of the surface brightness range 24-25 B arcsec-2 within which an average nuclear offset was determined. The objects can be identified from the VCC numbers given in the plots. Each image is 45 arcsecs on a side, except VCC1563 which is 1 arcmin. A detailed description is given in the text (Sect. 4).

A potential problem is the presence of globular clusters in the dwarfs. Deep HST images of Virgo dwarf ellipticals (dEs and dE,Ns) by Miller et al. (1998) show many globulars. The distinction between a faint nucleus and a globular may become impossible at this level of high resolution. However, our data do not go deep enough for this to be of any concern.

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© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000
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