TY - JOUR
T1 - Abundance determination from global emission-line SDSS spectra
T2 - exploring objects with high N/O ratios
AU - Pilyugin, L.S.
AU - Vilchez, J.M.
AU - Mattsson, Lars Mattias
AU - Thuan, T.X.
PY - 2012/4/1
Y1 - 2012/4/1
N2 - 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 Hii 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 Hii 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 Hii 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 ~0.2dex.
AB - 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 Hii 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 Hii 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 Hii 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 ~0.2dex.
U2 - 10.1111/j.1365-2966.2012.20420.x
DO - 10.1111/j.1365-2966.2012.20420.x
M3 - Journal article
SN - 0035-8711
VL - 421
SP - 1624
EP - 1634
JO - Royal Astronomical Society. Monthly Notices
JF - Royal Astronomical Society. Monthly Notices
IS - 2
ER -