Oxygen and Nitrogen in Leo A and GR 8

Abstract

We present elemental abundances for multiple H II regions in Leo A and GR 8 obtained from long-slit optical spectroscopy of these two nearby low-luminosity dwarf irregular galaxies. As expected from their luminosities, and in agreement with previous observations, the derived oxygen abundances are extremely low in both galaxies. High signal-to-noise ratio (S/N) observations of a planetary nebula in Leo A yield 12 + log(O/H) = 7.30 ± 0.05; calculations of the oxygen abundance in four H II regions in Leo A indicate 12 + log(O/H) = 7.38 ± 0.10. These results confirm that Leo A has one of the lowest ISM metal abundances of known nearby galaxies. Based on results from two H II regions with high S/N measurements of the weak [O III] λ4363 line, the mean oxygen abundance of GR 8 is 12 + log(O/H) = 7.65 ± 0.06; using empirical and semiempirical methods, similar abundances are derived for six other GR 8 H II regions. Similar to previous results in other low-metallicity galaxies, the mean log(N/O) = -1.53 ± 0.09 for Leo A and -1.51 ± 0.07 for GR 8. There is no evidence of significant variations in either O/H or N/O in the H II regions. The metallicity-luminosity relation for nearby (D < 5 Mpc) dwarf irregular galaxies with measured oxygen abundances has a mean correlation of 12 + log(O/H) = 5.67MB-0.151MB, with a dispersion in oxygen about the relationship of σ = 0.21. These observations confirm that gas-rich, low-luminosity galaxies have extremely low elemental abundances in the ionized gas phase of their interstellar media. Although Leo A has one of the lowest metal abundances of known nearby galaxies, detection of tracers of an older stellar population (RR Lyrae variable stars, horizontal branch stars, and a well-populated red giant branch) indicate that it is not a newly formed galaxy, as has been proposed for some other similar low-metallicity star-forming galaxies.