Are " Dwarf " Ellipticals Genuine Ellipticals?

We review the systematic properties of “dwarf” elliptical (dE) galaxies, focussing on the relation between “normal” and “dwarf” ellipticals. In recent years, this relation has been described as “dichotomy” – based essentially on a discontinuity in central surface brightness. We show that, outside of 300 pc from the centre, the Sérsic profile parameters vary continuously from “normal” to “dwarf” ellipticals. The “dichotomy” is indeed restricted to the very central part, where differences also exist among “normal” ellipticals (E). Bright, nucleated dE’s closely resemble “normal” E’s also in their clustering and flattening properties. They may be genuine ellipticals, having no present-day late-type dwarf precursor from which they could have been manufactured. The non-nucleated dE’s (”dwarf spheroidals”) may be a different breed. 1. What to call a dE galaxy? Elliptical galaxies are distinguished from spirals and late-type dwarfs by their smooth light distribution. Fainter than MBT ∼ −18 they divide up into two classes: compact ellipticals with high surface brightness, exemplified by M32, and diffuse ellipticals with low surface brightness, exemplified by the dwarf spheroidals in the Local Group (LG). Many different terms like dwarf ellipticals, dwarf spheroidals, spheroidals, or LSB galaxies are in use for the second class; there is no generally accepted definition. This led to some confusion. In particular, one debates whether faint ellipticals like M32 should be called dE. In our discussion we adopt the classification scheme worked out and illustrated in the Virgo cluster dwarf atlas of Sandage & Binggeli (1984), where the term “dwarf ellipticals” encompasses both local dwarf spheroidals and similar looking galaxies beyond the LG. Faint ellipticals with high surface brightness are referred to as ellipticals or compact ellipticals (cE) but never dwarf ellipticals. For a detailed discussion of the nomenclature issue see Binggeli (1994) and Kormendy & Bender (1994). Within the dwarf elliptical family there is a subtype classified as dwarf S0 (dS0). These galaxies are among the brightest dwarf ellipticals, being a very rare species. In the Virgo cluster, for instance, only 25 dS0’s are known as compared to 800 dE’s. The dS0’s show morphological characteristics of a bulgeto-disk transition which is typical for classical S0’s. However, a bar feature or simply high apparent flattening (Sandage & Binggeli 1984, panel 8) are other