Dwarf ellipticals span a range of at least 104 in luminosity (from MB -18 to -8) along a sequence of increasing mean surface brightness with increasing luminosity (see Sect. 2.2.2). At the bright end of this sequence, the relatively high surface brightness of a dE can mimic a normal elliptical, and quantitative analysis of surface brightness profiles is necessary to attempt a dE versus E distinction. This has proven difficult (see Sect 2.2.1). From a purely morphological (classificatory) point of view, there will always be a certain number of bright ``intermediate'' (E/dE) types, irrespective of a possible discontinuity in various measurable properties between E's and dE's (Binggeli and Cameron 1991; Prugniel 1994; Vader and Chaboyer 1994).
A similar problem exists for the dE versus Irr distinction at the faint end; in fact, for the whole range MB < -16. Dwarf irregulars in this range can appear very smooth and dE-like - presumably when they happen to be ``sleeping'', i.e. at a low or zero star formation rate. In the Virgo cluster, there is a broad dE/Irr class of galaxies comprising roughly 10% of the whole dwarf population, with increasing percentage faintwards (Sandage and Binggeli 1984; Sandage et al. 1985b). It has become clear over the years that this is not just a problem of classification (as it might be in the case of E versus dE): there appears to be a continuum of intrinsic properties such as gas content, metallicity, and star formation rate (Sect. 4) among dwarf galaxies. There are truly intermediate types which are probably in a transitional stage from Irr to dE (Sect. 7.6). A prototype dE/Irr in our neighborhood is the Phoenix system (van de Rydt et al. 1991). The Andromeda satellites NGC 205 and 185, too, are well-known ``peculiar'' dE's that contain dust and gas (e.g. Hodge 1971). Other, more distant examples of ``mixed morphology'' have been discussed by Sandage and Hoffman (1991) and Sandage and Fomalont (1993).
The following features are also relevant for the dE morphology (cf. Sandage and Binggeli 1984):
(1) Nuclei.
(2) Dwarf S0 types.
(3) Huge, low-surface brightness types.
Typical dwarf ellipticals, as well as some related types, are shown in
Fig. 1a-n.
Figure 2. Surface brightness profiles of five dwarf
ellipticals with prominent nuclei. The solid lines represent the sum of
a King profile to the outer regions (with parameters
rc, µ0, and
log(rt / rc)), and a central point
source (convolved with the approppriate PSF) fit to the inner regions
such that the observed surface brightness is nowhere exceeded. The
percentage of the total light contributed by the central light excess
(nucleus) with respect to the King fit is given under the heading
fex. From
Vader and Chaboyer
(1994).
Figure 1. A collection of dwarf galaxy members of the
Virgo cluster, adapted from
Binggeli (1994a).
The common scale is indicated on
top. Typical dwarf ellipticals in the left row (c,
f, i, l) along with the two ``dS0'' variants
(a, d) are confronted with a low-luminosity, compact E
(b), and with various types of smooth (g, h) or
clumpy (k, m) dwarf irregulars. Two intermediate
(transitional) types are also shown (e, n), the latter of
which is a ``huge, low-surface brightness'' type. The individual names
and types are as follows: a = NGC 4431 (dS0(5),N), b =
NGC 4486B (E1), c =
IC 3328 (dE1,N), d =
IC 3435 (dS0(8),N),
e = NGC 4344 (S pec, N:/BCD), f =
IC 3457 (dE4,N),
g = IC 3416 (ImIII), h =
UGC 7636 (ImIII-IV), i =
VCC 1661 (dE0,N), k =
IC 3453 (ImIII/BCD), l =
VCC 354
(dE0), m = VCC 1313 (BCD), n =
IC 3475 (ImIV or dE2 pec)