Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. III. Subpopulations: distributions, shapes, origins

摘要

From a quantitative analysis of 413 Virgo cluster early-type dwarf galaxies(dEs) with Sloan Digital Sky Survey imaging data, we find that the dE class canbe divided into multiple subpopulations that differ significantly in theirmorphology and clustering properties. Three dE subclasses are shaped like thickdisks and show no central clustering: (1) dEs with disk features like spiralarms or bars, (2) dEs with central star formation, and (3) ordinary, bright dEsthat have no or only a weak nucleus. These populations probably formed frominfalling progenitor galaxies. In contrast, ordinary nucleated dEs follow thepicture of classical dwarf elliptical galaxies in that they are spheroidalobjects and are centrally clustered like E and S0 galaxies, indicating thatthey have resided in the cluster since a long time, or were formed along withit. These results define a morphology-density relation within the dE class. Wefind that the difference in the clustering properties of nucleated dEs and dEswith no or only a weak nucleus is not caused by selection biases, as opposed topreviously reported suggestions. The correlation between surface brightness andobserved axial ratio favors oblate shapes for all subclasses, but ourderivation of intrinsic axial ratios indicates the presence of at least sometriaxiality. We discuss possible interrelations and formation mechanisms(ram-pressure stripping, tidally induced star formation, harassment) of thesedE subpopulations.