* Astronomy

Title: Interstellar Medium Oxygen Abundances of Dwarf Irregular Galaxies in Centaurus A and Nearby Groups
Authors: Henry Lee (1,2), D. B. Zucker (3), E. K. Grebel (4) ((1) Gemini Observatory, (2) Univ. of Minnesota, (3) Institute of Astronomy, Univ. of Cambridge, (4) Astronomical Institute, Univ. of Basel)

We present results of optical spectroscopy of 35 H II regions from eight dwarf galaxies in the Centaurus A group. (O III)4363 A is detected in ESO272-G025 and ESO324-G024, and direct oxygen abundances of 12+log(O/H) = 7.76 ±0.09 and 7.94 ±0.11 are derived, respectively. For the remaining galaxies, abundances are derived using common bright-line methods. To compare the influence of group environments on dwarf galaxies, we have also gathered data for additional dwarf irregular galaxies from the Cen A and the Sculptor groups from the literature. We have examined possible relationships between oxygen abundance, gas fraction, effective chemical yield, and tidal indices. Despite large positive tidal indices for a number of Cen A dwarfs in the present sample, there is no clear separation between galaxies with positive tidal indices and galaxies with negative tidal indices in the luminosity-metallicity, metallicity-gas fraction, and metallicity-tidal index diagrams. The H I surface mass density decreases with increasing positive tidal index, which is expected in strong tidal encounters. There are no strong trends between oxygen abundances or yields and projected distances of galaxies within their respective groups. We also present spectra for 13 H II regions in three nearby dwarf irregular galaxies: DDO 47, NGC 3109, and Sextans B. For DDO 47, the (O III)4363 oxygen abundance (7.92 ±0.06) for the H II region SHK91 No. 18 agrees with recently published values. For Sextans B, the (O III)4363 oxygen abundance (7.80 ±0.13) for H II region SHK91 No. 5 agrees with published work in which O^+ abundances were determined entirely from (O II)7320,7330 fluxes.

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