High N/O ratio at high redshift as a result of a strong burst of star formation and differential galactic winds

¹ Dipartimento di Fisica, Università degli Studi di Trieste, via Tiepolo 11, I-34143 Trieste, Italy
² INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34143 Trieste, Italy
³ INFN, Sezione di Trieste, via Valerio 2, I-34134 Trieste, Italy
⁴ Institute for Fundamental Physics of the Universe, via Beirut, 2, I-34151 Trieste, Italy
⁵ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁷ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK

^⋆ Corresponding author: federico.rizzuti@inaf.it

Received: 3 December 2024
Accepted: 25 February 2025

Abstract

Context. Recent observations by JWST have revealed supersolar ¹⁴N abundances in galaxies at very high redshift. On the other hand, these galaxies show subsolar metallicity. The observed N/O ratios are difficult to reproduce in the framework of chemical evolution models for the Milky Way.

Aims. Our aim is to reproduce these high N/O ratios with chemical evolution models, assuming different histories of star formation triggering galactic winds, coupled with detailed nucleosynthesis prescriptions for ¹⁴N, ¹²C, ¹⁶O, and ⁵⁶Fe.

Methods. We computed several models for small galaxies (10⁹−10¹⁰ M_⊙) with a high star formation efficiency and strong galactic winds. These winds are assumed to be differential, mainly carrying out the products of the explosion of core-collapse supernovae.

Results. We find that only models with high star formation rates, a normal initial mass function, and differential galactic winds can reproduce the observed chemical abundances. We also find that, with the same assumptions about star formation and galactic winds, but with a very rapid formation resulting from fast gas infall, we can also reproduce the estimated ages of these objects. We find no necessity to invoke peculiar nucleosynthesis from population III stars, very massive stars, and supermassive stars.