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

Abstract

We present Hα imaging and optical spectrophotometry of H II regions in low surface brightness dwarf irregular DDO 154. The possesses a very small population of faint discrete H II regions and larger diffuse H II regions and ionized shells. We confirm 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 previous characterization of DDO 154 as a quiescent dwarf irregular galaxy. Spectrophotometry of two of brightest H II regions yields a relatively low oxygen abundance of 0.055 ± 0.008 (O/H)⊙, in agreement with previous determination by van Zee et al., and in accordance with 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 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 neutral gas is chemically homogeneous, we derive an effective oxygen yield of roughly 50% of solar value, a value that is close to theoretically favored values for true oxygen yield. Thus, it is possible that DDO 154 is evolving nearly as a closed system. On other hand, if abundances in extended H I disk are lower than in H II regions, derived value of effective yield has been artificially inflated, and DDO 154 may have experienced significant loss of metal-enriched gas.