On the origin of nitrogen at low metallicity

Abstract

Understanding the evolution of the N/O ratio in the interstellar medium (ISM) of galaxies is essential if we are to complete our picture of the chemical evolution of galaxies at high redshift, since most observational calibrations of O/H implicitly depend upon the intrinsic N/O ratio. The observed N/O ratio, however, shows large scatter at low O/H, and is strongly dependent on galactic environment. We show that several heretofore unexplained features of the N/O distribution at low O/H can be explained by the N seen in metal-poor galaxies being mostly primary nitrogen that is returned to the ISM via pre-supernova winds from rapidly rotating massive stars ($M\gtrsim 10$ M${}_{\odot }$, $v/v_{\mathrm{crit}}\gtrsim 0.4$). This mechanism naturally produces the observed N/O plateau at low O/H. We show that the large scatter in N/O at low O/H also arises naturally from variations in star-formation efficiency. By contrast, models in which the N and O come primarily from supernovae provide a very poor fit to the observed abundance distribution. We propose that the peculiar abundance patterns we observe at low O/H are a signature that dwarf galaxies retain little of their SN ejecta, leaving them with abundance patterns typical of winds.

19 pages, 14 figures, 4 tables, accepted for publication in MNRAS

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• The impact of carbon and oxygen abundances on the metal-poor initial mass function
• Nitrogen evolution in the halo, thick disc, thin disc and bulge of the Galaxy
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• Strong nebular HeII emission induced by He${}^{+}$ ionizing photons escaping through the clumpy winds of massive stars
• Progenitors of Long-Duration Gamma-ray Bursts
• Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840
• A constant upper luminosity limit of cool supergiant stars down to the extremely low metallicity of I Zw 18
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