The Abundances of Carbon and Nitrogen in I ZW 18

Abstract

We present IUE and McDonald IDS spectrophotometry of the NW H II region in the most oxygen-poor galaxy known, I Zw 18. The C III] λ1909 and [N II] λ6583 emission lines were detected for the first time in this galaxy. Using a photoionization model for the H II region, we derive 12 + log N(X)/N(H) = 6.94 +/- 0.24 for C, 5.99 +/- 0.16 for N, and 7.24 +/- 0.07 for O. The C/O and N/O ratios found for I Zw 18 exceed the values found in the Magellanic Clouds and other well-observed oxygen-poor irregular galaxies and are more similar to those in the Sun and nearby Galactic H II regions. One interpretation is that I Zw 18 may be older and more chemically evolved then previously believed, and that its extremely low O/H results from escape of O-rich supernova ejecta from the galaxy or from dilution by accretion of primordial gas. Alternatively I Zw 18 may have formed from a preenriched intergalactic medium. Composite photoionization models, with a range of ionization parameters or ionizing star temperatures, may be required to fit the spectrum of I Zw 18, in particular the high [O III] temperature. We discuss spectroscopic diagnostics for such models and the implications for measurements of the primordial helium abundance.