H [CSC]ii[/CSC] Regions and Abundances in the “Dark Galaxy” DDO 154 and the Chemical Evolution of Dwarf Irregular Galaxies

Abstract

We present H alpha imaging and optical spectrophotometry of H II regionsin the low surface brightness dwarf irregular galaxy DDO 154. The galaxypossesses a very small population of faint discrete H II regions andlarger diffuse H II regions and ionized shells. We confirm the very lowstar formation rate and extremely long gas consumption times reportedpreviously by van Zee, Haynes, & Salzer. The current star formationrate is similar to2-4 times lower than its average past rate, confirmingthe previous characterization of DDO 154 as a ``quiescent{''} dwarfirregular galaxy. Spectrophotometry of two of the brightest H II regionsyields a relatively low oxygen abundance of 0.055 +/- 0.008 (O/H)., inagreement with the previous determination by van Zee et al., and inaccordance with the previously determined metallicity-luminosityrelationship for dwarf irregular galaxies. We also find an N/O ratio of0.037 +/- 0.003, which is marginally higher than the typical value of0.025 found in low-metallicity blue compact galaxies. Although DDO 154has been labeled ``the dark galaxy{''} and is a prototype for lowsurface brightness galaxies with large H I content, its chemicalabundances are consistent with an average, low-mass, dwarf irregulargalaxy. Assuming that the neutral gas is chemically homogeneous, wederive an effective oxygen yield of roughly 50% of the solar value, avalue that is close to the theoretically favored values for the trueoxygen yield. Thus, it is possible that DDO 154 is evolving nearly as aclosed system. On the other hand, if the abundances in the extended H Idisk are lower than in the H II regions, the derived value of theeffective yield has been artificially inflated, and DDO 154 may haveexperienced significant loss of metal-enriched gas.