The dwarf galaxy population of the Virgo cluster is dominated by dwarf ellipticals. These may have been accreted by the cluster as different types of galaxies, and may have been transformed into dEs through some interaction with the ICM or with other cluster galaxies. A significant fraction (~ 10%) of the DGs belongs to the dI class and shows, in many cases, signs of current star formation.
Various authors proposed that at least the dEs result from an accreted population of progenitor galaxies; presently the nature of these predecessor objects is not clear, nor is there a visible reservoir of such galaxies in a pre-accretion phase. We therefore must await a better explanation for the dEs, or another idea about their origin.
Our extensive studies of late-type dwarf galaxies in the VC show that the proposed scenario relating BCDs to LSB dwarfs (lazy galaxies that sometimes flare-up) is not tenable. There is no way by which an Im IV-V object can become a BCD, and vice-versa. However, there are cluster-related mechanisms by which a galaxy may lose part of its ISM; these are harassment, tidal interactions, and ram-pressure sweeping, and may be more relevant to dwarf galaxies because of their shallower gravitational potential.
Acknowledgements
I am grateful to Ana Beatriz Heller and Elchanan Almoznino for major contributions to the study of the Virgo cluster dwarf irregular populations. At various stages of this project G. Lyle Hoffman helped with data and Ed Salpeter discussed some of the results. Data and remarks by John Salzer and Liese van Zee are greatly appreciated. Part of the work reported here was done while I was a sabbatical visitor at the Space Telescope Science Institute; I am grateful to Bob Williams, its past Director for facilitating this extended stay. Research at the Wise Observatory is supported by grants from the Israel Science Foundation and from the Austrian Friends of Tel Aviv University.