Uncovering additional clues to galaxy evolution. II. The environmental impact of the Virgo Cluster on the evolution of dwarf irregular galaxies

The impact of the cluster environment on the evolution of dwarf galaxies is investigated by comparing the properties of a sample of dwarf irregular galaxies (dI's) in the Virgo Cluster with a control sample of nearby (field'') dI's having oxygen abundances derived from [O III] lambda4363 measurements and measured distances from resolved stellar constituents. Spectroscopic data are obtained for H II regions in 11 Virgo dI's distributed in the central and outer regions of the cluster. To ensure that oxygen abundances are derived in a homogeneous manner, oxygen abundances for field and Virgo dI's are computed using the bright-line method and compared with abundances directly obtained from [O III] lambda4363, where available. They are found to agree to within about 0.2 dex, with no systematic offset. At a given optical luminosity, there is no systematic difference in oxygen abundance between the sample of Virgo dI's and the sample of nearby dI's. However, five of the 11 Virgo dI's exhibit much lower baryonic gas fractions than field dI's at comparable oxygen abundances. Using field dI's as a reference, a gas-deficiency index for dI's is constructed, making it possible quantitatively to identify which galaxies have lost gas. For the Virgo sample, some of the dwarfs are gas-deficient by a factor of 30. The gas deficiency correlates roughly with the X-ray surface brightness of the intracluster gas. Ram pressure stripping can best explain the observed gas-poor dI's in the cluster sample. Together with the lack of significant fading and reddening of the gas-poor dI's compared with gas-normal dI's, these observations suggest that the gas-poor dI's in Virgo have recently encountered the intracluster medium for the first time. Faded remnants of gas-poor dI's in Virgo will resemble bright dwarf elliptical galaxies currently seen in the cluster core.