Comparative photometric parameters of dwarf irregular and elliptical galaxies in the Virgo Cluster - Two different classes of dwarf galaxies?

Abstract

The possible evolutionary relationships of dwarf irregular (dI) and dwarf elliptical (dE) galaxies is explored with a data base that consists of 21 cm data, infrared photometry, and CCD multicolor surface photometry of both dIs and dEs in the Virgo cluster. In general, dIs are at or slightly below the same surface-brightness level as dEs, yet they are significantly bluer. To have the same optical and IR color distribution as the dEs requires that dIs fade by 1.5 mag in the blue. This will in turn produce a population of dwarf galaxies that will have average surface brightness below the detection threshold of conventional photographic plates. It is concluded that the progenitors of the brighter dE galaxies in the Virgo cluster are not to be found among the current generation of dIs with the exception that some of the larger blue compact dwarf galaxies are probably gas-rich analogs to dEs. Furthermore, it is probable that dIs form a parallel sequence of dwarf galaxies with surface mass densities significantly lower than the dEs.