Metallicity gradients in Local Universe galaxies: time evolution and effects of radial migration

TL;DR

This study investigates the evolution of metallicity gradients in nearby spiral galaxies using nebular abundances, revealing that gradients tend to flatten over time and are influenced by radial migration, with implications for galaxy evolution models.

Contribution

First comprehensive analysis of radial metallicity gradients and stellar migration in multiple local galaxies using direct nebular abundances, providing new constraints on galaxy evolution.

Findings

PN gradients are flatter or equal to Hii region gradients.

Oxygen in PNe reliably traces past ISM metallicity.

Total metal content increases over time in all studied galaxies.

Abstract

Our knowledge of the shape of radial metallicity gradients in disc galaxies has recently improved. Conversely, the understanding of their time evolution is more complex, since it requires analysis of stellar populations with different ages, or systematic studies of galaxies at different redshifts. In the Local Universe, Hii regions and planetary nebulae (PNe) are important tools to investigate it. We present an in-depth study of all nearby spiral galaxies (M33, M31, NGC300, and M81) with direct-method nebular abundances of both populations. For the first time, we also evaluate the radial migration of PN populations. We analyse Hii region and PN properties to: determine whether oxygen in PNe is a reliable tracer for past interstellar medium (ISM) composition; homogenise the published datasets; estimate the migration of the oldest stellar populations; determine the overall chemical enrichment and slope evolution. We confirm that oxygen in PNe is a reliable tracer for the past ISM metallicity. We find that PN gradients are flatter than or equal to those of Hii regions. When radial motions are negligible, this result provides a direct measurement of the time evolution of the gradient. For galaxies with dominant radial motions, we provide upper limits on the gradient evolution. Finally, the total metal content increases with time in all target galaxies, with early morphological type having a larger increment Delta(O/H) than late-type galaxies. Our findings provide important constraints to discriminate among different galactic evolutionary scenarios, favouring cosmological models with enhanced feedback from supernovae. The advent of extremely large telescopes will allow us to include galaxies in a wider range of morphologies and environments, thus putting firmer constraints to galaxy formation and evolution scenarios.