Journal Article

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

L. S. Pilyugin, J. M. Vílchez, L. Mattsson and T. X. Thuan

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 421, issue 2, pages 1624-1634
Published in print April 2012 | ISSN: 0035-8711
Published online March 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.20420.x
Abundance determination from global emission-line SDSS spectra: exploring objects with high N/O ratios

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We have compared the oxygen and nitrogen abundances derived from global emission-line Sloan Digital Sky Survey (SDSS) spectra of galaxies using (1) the Te method and (2) two recent strong-line calibrations: the ON and NS calibrations. Using the Te 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 Te 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 Te-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 ∼0.2 dex.

Keywords: ISM: abundances; H ii regions; galaxies: abundances

Journal Article.  7703 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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