Effects of secular growth and mergers on the evolution of metallicity gradients and azimuthal variations in a Milky Way-like galaxy

TL;DR

This study uses simulations to show that metallicity gradients in a Milky Way-like galaxy are established early, persist through mergers, and are mainly shaped by inside-out growth and radial migration, with mergers having limited impact.

Contribution

It provides new insights into how metallicity gradients evolve over time, highlighting the limited effect of mergers and the importance of internal processes.

Findings

Negative metallicity gradients are present early and persist.

Major mergers have a short-lived and minimal impact on metallicity distributions.

Radial migration and inside-out growth primarily shape metallicity profiles.

Abstract

We analyze the evolution of the radial profiles and the azimuthal variations of the stellar metallicities from the Vintergatan simulation of a Milky Way-like galaxy. We find that negative gradients exist as soon as the disk settles at high redshift, and are maintained throughout the long term evolution of the galaxy, including during major merger events. The inside-out growth of the disk and an overall outward radial migration tend to flatten these gradients in time. Major merger events only have a moderate and short-lived imprint on the [Fe/H] distributions with almost no radial dependence. The reason lies in the timescale for enrichment in Fe being significantly longer than the duration of the starbursts episodes, themselves slower than dynamical mixing during typical interactions. It results that signatures of major mergers become undetectable in [Fe/H] only a few Myr after pericenter passages. We note that considering other tracers like the warm interstellar medium, or monitoring the evolution of the metallicity gradient as a single value instead of a radial full profile could lead to different interpretations, and warn against an oversimplification of this complex problem.