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H II Regions and Abundances in the “Dark Galaxy” DDO 154 and the Chemical Evolution of Dwarf Irregular Galaxies

We present Hα imaging and optical spectrophotometry of H II regions in the low surface brightness dwarf irregular galaxy DDO 154. The galaxy possesses a very small population of faint discrete H II regions and larger diffuse H II regions and ionized shells. We confirm the very low star formation rate and extremely long gas consumption times reported previously by van Zee, Haynes, & Salzer. The current star formation rate is ∼2–4 times lower than its average past rate, confirming the previous characterization of DDO 154 as a "quiescent" dwarf irregular galaxy. Spectrophotometry of two of the brightest H II regions yields a relatively low oxygen abundance of 0.055 ± 0.008 (O/H)⊙, in agreement with the previous determination by van Zee et al., and in accordance with the previously determined metallicity-luminosity relationship for dwarf irregular galaxies. We also find an N/O ratio of 0.037 ± 0.003, which is marginally higher than the typical value of 0.025 found in low-metallicity blue compact galaxies. Although DDO 154 has been labeled "the dark galaxy" and is a prototype for low surface brightness galaxies with large H I content, its chemical abundances are consistent with an average, low-mass, dwarf irregular galaxy. Assuming that the neutral gas is chemically homogeneous, we derive an effective oxygen yield of roughly 50% of the solar value, a value that is close to the theoretically favored values for the true oxygen yield. Thus, it is possible that DDO 154 is evolving nearly as a closed system. On the other hand, if the abundances in the extended H I disk are lower than in the H II regions, the derived value of the effective yield has been artificially inflated, and DDO 154 may have experienced significant loss of metal-enriched gas.