The nitrogen‐to‐oxygen evolution in galaxies: the role of the star formation rate

ABSTRACT The main objective of this work is to check if the star formation efficiency plays a relevant role in the evolution of the relative abundance N/O. In order to explore this idea, we analyse the evolution of the nitrogen‐to‐oxygen ratio as predicted by a set of computed theoretical models. These models consist of simulated galaxies with different total masses which are evolved assuming different collapse time‐scales and different star formation efficiencies. The combinations of these two parameters produce different star formation histories, which in turn have, as we show, an important impact on the resulting N/O ratio. Since we want to check the effect of variations in these efficiencies on our model results, the same stellar yield sets are used for all of them. The selected yields have an important primary nitrogen contribution coming from low‐ and intermediate‐mass stars, which implies that N is ejected with a certain delay with respect to O. It allows us to obtain, as we demonstrate, a dispersion of results in the N/O–O/H plane when star formation efficiencies vary which is in general agreement with observations. The model results for the N/O abundance ratio are in good agreement with most observational data trends. In particular, the behaviour shown by the extragalactic H  ii regions is well reproduced with present‐time resulting abundances. Furthermore, the low N/O values estimated for high‐redshift objects, such as those obtained for damped Lyman α galaxies, as well as the higher (and constant) values of N/O observed for irregular and dwarf galaxies or halo stars, can be simultaneously obtained with our models at the same low oxygen abundances 12 + log(O/H) ∼ 7. We therefore conclude that, even though there seems to be a general belief that abundance ratios depend mostly on stellar yields, these are not the only parameter at work when both elements are ejected by stars of different mass ranges, and that differences in the star formation history of galaxies and regions within them are a key factor to explain the data in the N/O–O/H plane.