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H. Lee, M.L. McCall (York University), M.G. Richer (UNAM)
Chemical and structural properties of Virgo Cluster dwarf irregular galaxies are compared with those of Local Group dwarf irregulars to determine the extent of the effect of cluster environment on dwarf galaxies. A sample of seven Virgo dwarf irregular galaxies with luminosities -15 \ga MB \ga -16 was selected. Results obtained from recent optical long-slit spectrophotometry and broad-band photometry of these Virgo dwarf galaxies are presented. The mean oxygen abundances for the seven Virgo dwarf irregulars are found to be in the range 12 + \log(O/H) = 8.0 to 8.3, which are consistent with oxygen abundances for Local Group dwarf irregulars at similar luminosities. Computed N/O ratios for the Virgo galaxies are also similar to the Local Group sample. Measured star-forming properties (e.g. H\alpha luminosities, the number of ionizing photons) appear to be at the lower end of the range of properties compiled for extragalactic H~II regions. Yet, five of the seven Virgo dwarf galaxies exhibit much lower MH I/LB ratios than found for Local Group dwarf galaxies at similar luminosities and metallicities. While the higher-density Virgo environment has affected gas content, chemical evolution has led to similar metal fractions for a given stellar mass in both Local Group and Virgo dwarf galaxies. For the Virgo sample, this presents the situation whereby the presently observed mass--metallicity relationship is maintained, but the H~I gas mass is reduced much farther than what is observed in the Local Group. A way of achieving these conditions may be the sudden removal of gas arising from intracluster medium ram-pressure stripping.