University of Kansas
Monday, March 26, 2012
Metallicity values and mass-metallicity relations determined in large surveys of star-forming galaxies are important constraints to models of galaxy evolution. However, the techniques frequently employed for estimating metallicities are subject to a variety of systematic errors. This project studies in detail the abundances and abundance determination methods applied to one unique type of star-forming galaxy, known as "Green Peas". The Peas are characterized by low mass, low metallicity, strong [O III] λλ4959, 5007, and redshifts in the range of ~0.1-0.4. He II λ4686 is identified for the first time in the spectra of nine of the galaxies and measured λ4686/Hβ ratios are consistent with photoionization by Wolf-Rayet stars. This implies that the spectral energy distribution of the ionizing source is harder than typically seen in star-forming galaxies. Emission-line intensities are used to determine Te-based abundances of O, N, Ne, S, and He in a majority of the galaxies. The average N/O is evidence of a modest nitrogen enhancement compared with other low-metallicity galaxies possibly due to Wolf-Rayet stars or to intermediate-mass stars during a previous quiescent period. Photoionization by Wolf-Rayet stars raises questions about the validity of the strong-line methods based on [N II]/Hα, [N II]/[O III], or [N II]/[S II], often used for estimating metallicity, as these line ratios are known to be ionization-sensitive. It is shown that ionization, low metallicity, and the small variation in important line ratios in the Green Pea spectra all affect the behavior of one or more of the N2, O3N2, N2O2 and N2S2 strong-line methods. The reported increase of N/O with O/H in the Green Peas appears to be a bias introduced by combining N2 with N2S2. N2O2 does not give valid results in the Green Peas, while N2 and N2S2 do, although the calibrations of the N2 and N2S2 methods based on Green Pea abundances are different from the existing calibrations of the methods that are based primarily on abundances in extragalactic H II regions and H II galaxies.