Nitrogen and oxygen abundances in the Local Universe

Fiorenzo Vincenzo, Francesco Belfiore, Roberto Maiolino, Francesca Matteucci, Paolo Ventura

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    Abstract

    We present chemical evolution models aimed at reproducing the observed (N/O) versus (O/H)
    abundance pattern of star-forming galaxies in the Local Universe. We derive gas-phase abundances
    from Sloan Digital Sky Survey (SDSS) spectroscopy and a complementary sample of
    low-metallicity dwarf galaxies, making use of a consistent set of abundance calibrations. This
    collection of data clearly confirms the existence of a plateau in the (N/O) ratio at very low
    metallicity, followed by an increase of this ratio up to high values as the metallicity increases.
    This trend can be interpreted as due to two main sources of nitrogen in galaxies: (i) massive
    stars, which produce small amounts of pure primary nitrogen and are responsible for the (N/O)
    ratio in the low-metallicity plateau; (ii) low- and intermediate-mass stars, which produce both
    secondary and primary nitrogen and enrich the interstellar medium with a time delay relative
    to massive stars, and cause the increase of the (N/O) ratio. We find that the length of the
    low-metallicity plateau is almost solely determined by the star formation efficiency, which
    regulates the rate of oxygen production by massive stars. We show that, to reproduce the high
    observed (N/O) ratios at high (O/H), as well as the right slope of the (N/O) versus (O/H) curve,
    a differential galactic wind – where oxygen is assumed to be lost more easily than nitrogen
    – is necessary. No existing set of stellar yields can reproduce the observed trend without
    assuming differential galactic winds. Finally, considering the current best set of stellar yields,
    a bottom-heavy initial mass function is favoured to reproduce the data.
    Original languageEnglish
    Article numberstw532
    Pages (from-to)3466-3477
    Number of pages12
    JournalMonthly Notices of the Royal Astronomical Society
    Volume458
    Issue number4
    Early online date10 Mar 2016
    DOIs
    Publication statusPublished - 1 Jun 2016

    Keywords

    • stars: abundances
    • ISM: abundances
    • ISM: evolution
    • galaxies: abundances
    • galaxies: evolution
    • galaxies: ISM

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