5. On the taxonomy of dwarf elliptical galaxies
Dwarf galaxies come in two basic brands: dwarf ellipticals (dEs) and dwarf irregulars (dIrrs). The common view is that dEs are spheroids and dIrrs are disks. In the case of irregulars, the disk nature is clearly indicated by the typical rotation pattern found in HI radio data (at least for dIrrs more luminous than ). Owing to the absence of gas and hence the lack of an easily accessible kinematic tracer, the situation is less clear for dEs. The basic evidence for their spheroidal nature is purely statistical: the flattening distribution of this dwarf type is very similar to the one shown by normal ellipticals, which is distinctly different from disk galaxies (Ryden & Terndrup 1994; Binggeli & Popescu 1995). Supporting evidence for the spheroidal nature of dEs comes from spectroscopy. The measurements so far clearly show a lack of rotation (Paltoglou & Freeman 1987; Bender & Nieto 1990; Bender et al. 1991).
However, there are strong indications suggesting that at least part of the dwarf galaxies classified "dE" might be genuine disk galaxies:
(1) There is a general photometric kinship between dEs and low-luminosity late-type galaxies (dIrrs and spirals of type Sd and Sm); e.g. both have roughly exponential surface brightness profiles (e.g. Lin & Faber 1983; Binggeli & Cameron 1993). Coupled with the general morphology-density relation of dwarf galaxies (Einasto et al. 1974; Binggeli et al. 1987), this has fostered the idea that a significant fraction of the dwarf ellipticals might be the dead ends of evolved irregulars and low-luminosity spirals (Lin & Faber 1983; Kormendy 1985; Ferguson & Sandage 1989; see Ferguson & Binggeli 1994 for a broader discussion). In case of a sufficiently soft evolutionary mechanism such as ram pressure stripping, a dwarf galaxy could indeed have preserved its disk structure while loosing its gas.
(2) Sandage & Binggeli (1984), in their morphological work on Virgo cluster dwarfs, introduced the class of dwarf S0 galaxies. Objects classified dS0, while being much rarer than dEs and confined to a bright magnitude range, are characterized either by a hint of a two-component (S0-like) structure, or some other peculiar feature like boxyness, bar-like feature, or extreme flattening (see also BC91). The flattening distribution of dS0s is typical for disk galaxies (Binggeli & Popescu 1995).
(3) More evidence for the structural diversity of dEs is provided by Ryden et al. (1999). From an isophotal analysis of a sample of Virgo cluster dEs and dS0s these authors found the same range and frequency of "boxy" versus "disky" distortions from ellipticity as in normal ellipticals (e.g. Bender et al. 1989). Clearly, some of these early-type dwarf galaxies will be disk galaxies, or at least have a disk component embedded.
The fourth and latest indication for a disk in a dE of course is the spiral in IC3328. Despite its classification as dE, this galaxy clearly is a mis-classified dS0. We would like to emphasize that IC3328 does not resemble a "dwarf spiral", a new galaxy type proposed by Schombert et al. (1995), as it lacks a bulge and shows no obvious signs of gas and dust.
It is very likely that more dS0s will be identified among the bright round dEs in the future by means of a careful study of the 2-D light distribution.
© European Southern Observatory (ESO) 2000
Online publication: June 20, 2000