Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios

We have compared the oxygen and nitrogen abundances derived from global emission-line Sloan Digital Sky Survey (SDSS) spectra of galaxies using (1) the T-e method and (2) two recent strong-line calibrations: the ON and NS calibrations. Using the T-e method, anomalously high N/O abundance ratios have been found in some SDSS galaxies. To investigate this, we have Monte Carlo simulated the global spectra of composite nebulae by a mix of spectra of individual components, based on spectra of well-studied H II regions in nearby galaxies. We found that the T-e method results in an underestimated oxygen abundance (and hence in an overestimated nitrogen-to-oxygen ratio) if H II regions with different physical properties contribute to the global spectrum of composite nebulae. This effect is somewhat similar to the small-scale temperature fluctuations in H II regions discussed by Peimbert. Our work thus suggests that the high T-e-based N/O abundance ratios found in SDSS galaxies may not be real. However, such an effect is not expected to be present in dwarf galaxies since they generally have a uniform chemical composition. The ON and NS calibrations give O and N abundances in composite nebulae which agree with the mean luminosity-weighted abundances of their components to within similar to 0.2 dex.