Uncovering Additional Clues to Galaxy Evolution. II. The Environmental Impact of the Virgo Cluster on the Evolution of Dwarf Irregular Galaxies

Abstract

The impact of the cluster environment on the evolution of dwarf galaxies is investigated by comparing the properties of a sample of dwarfirregular galaxies (dI's) in the Virgo Cluster with a control sample ofnearby (``field'') dI's having oxygen abundances derived from [O III]λ4363 measurements and measured distances from resolved stellarconstituents. Spectroscopic data are obtained for H II regions in 11Virgo 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 thebright-line method and compared with abundances directly obtained from[O III] λ4363, where available. They are found to agree to withinabout 0.2 dex, with no systematic offset. At a given optical luminosity,there is no systematic difference in oxygen abundance between the sampleof Virgo dI's and the sample of nearby dI's. However, five of the 11Virgo dI's exhibit much lower baryonic gas fractions than field dI's atcomparable oxygen abundances. Using field dI's as a reference, agas-deficiency index for dI's is constructed, making it possiblequantitatively to identify which galaxies have lost gas. For the Virgosample, some of the dwarfs are gas-deficient by a factor of 30. The gasdeficiency correlates roughly with the X-ray surface brightness of theintracluster gas. Ram pressure stripping can best explain the observedgas-poor dI's in the cluster sample. Together with the lack ofsignificant fading and reddening of the gas-poor dI's compared withgas-normal dI's, these observations suggest that the gas-poor dI's inVirgo have recently encountered the intracluster medium for the firsttime. Faded remnants of gas-poor dI's in Virgo will resemble brightdwarf elliptical galaxies currently seen in the cluster core.