Evolution of N/O ratios in galaxies from cosmological hydrodynamical simulations

Abstract

We study the redshift evolution of the gas-phase O/H and N/O abundances, both (i) for individual ISM regions within single spatially-resolved galaxies and (ii) when dealing with average abundances in the whole ISM of many unresolved galaxies. We make use of a cosmological hydrodynamical simulation including detailed chemical enrichment, which properly takes into account the variety of different stellar nucleosynthetic sources of O and N in galaxies. We identify 33 galaxies in the simulation, lying within dark matter halos with virial mass in the range 10 11 M DM 10 13 M and reconstruct how they evolved with redshift. For the local and global measurements, the observed increasing trend of N/O at high O/H can be explained, respectively, (i) as the consequence of metallicity gradients which have settled in the galaxy interstellar medium, where the innermost galactic regions have the highest O/H abundances and the highest N/O ratios, and (ii) as the consequence of an underlying average mass-metallicity relation that galaxies obey as they evolve across cosmic epochs, where -at any redshift -less massive galaxies have lower average O/H and N/O ratios than the more massive ones. We do not find a strong dependence on the environment. For both local and global relations, the predicted N/O-O/H relation is due to the mostly secondary origin of N in stars. We also predict that the O/H and N/O gradients in the galaxy interstellar medium gradually flatten as functions of redshift, with the average N/O ratios being strictly coupled with the galaxy star formation history. Because N production strongly depends on O abundances, we obtain a universal relation for the N/O-O/H abundance diagram whether we consider average abundances of many unresolved galaxies put together or many abundance measurements within a single spatially-resolved galaxy.

Influence of the Void Environment on Chemical Abundances in Dwarf Galaxies and Implications for Connecting Star Formation and Halo Mass

“…8 and 9 show that there is no significant difference in the abundance ratio relationships between void dwarf galaxies and dwarf galaxies in denser regions. This supports the results found by Douglass & Vogeley (2017b) and Vincenzo & Kobayashi (2018). The large-scale environment does not appear to have an influence in the nucleosynthesis of nitrogen in dwarf galaxies.…”

CHAOS IV: Gas-phase Abundance Trends from the First Four CHAOS Galaxies

“…In between these scenarios, continuous star formation with roughly 250 Myr between bursts will result in N and O increasing in lockstep, dependent on the elemental yields. The coupling of the N/O plateau with galaxy SFH is also reported by cosmological hydrodynamical simulations of individual regions within spatially-resolved galaxies (Vincenzo & Kobayashi 2018). In these simulations, asymptotic giant branch (AGB) stars contribute significant N at low O/H, but the exact value of the primary N/O plateau will vary from galaxy to galaxy according to the relative contributions from SNe and AGB stars, as determined by their galaxy formation time and SFH.…”

AGB dust and gas ejecta in extremely metal-poor environments

“…For example, the importance of studying the yields of the CNO elements from AGB stars has been recently outlined by Vincenzo & Kobayashi (2018a), in a paper focused on the reconstruction of the star formation history (SFH) of galaxies, based on the CNO trends observed in the interstellar medium. On the other hand, Vincenzo & Kobayashi (2018b) showed the importance of taking into account all the stellar sites of CNO nucleosynthesis, including AGB stars, to perform detailed chemo-dynamical simulations on cosmological scales, in which the N/O vs O/H trends are used to reconstruct the evolution of galaxies with redshift.…”