Timing the Milky Way bar formation and the accompanying radial migration episode

TL;DR

This study uses metallicity profiles and numerical simulations to date the Milky Way's bar formation to 8-10 Gyr ago, linking it to radial migration episodes and resonance effects that shaped the galaxy's structure.

Contribution

It provides new constraints on the timing of the Milky Way's bar formation and its impact on stellar radial migration, supported by detailed metallicity profile analysis and simulations.

Findings

The metallicity profile shows a break at 10-12 kpc linked to the bar's Outer Lindblad Resonance.

The bar formed 8-10 Gyr ago, with its slowdown ending 6-7 Gyr ago.

Limited radial migration occurs after the initial episode, affecting metallicity dispersion.

Abstract

We derive the metallicity profile of the Milky Way low-$\alpha$ disc population from 2 to 20 kpc from the Galactic centre in 1 Gyr age bins using the astroNN catalogue, and show that it is highly structured, with a plateau between 4 and 7 kpc and a break at 10-12 kpc. We argue that these features result from the two main bar resonances, the corotation and the Outer Lindblad Resonance (OLR), respectively. We show that the break in the metallicity profile is most visible in stars having 7-8 Gyr, reaching an amplitude of about 0.4 dex, and is the signpost of the position of the bar OLR. The bar formation was accompanied by an episode of radial migration triggered by its slowing down and is responsible for spreading old metal-rich stars up to the OLR. The data show that the slowdown of the bar ended 6-7 Gyr ago. Based on numerical simulations that reproduce well the break observed in the metallicity profile, we argue that this implies that the bar formed in our Galaxy 8-10 Gyr ago. Analysis of the metallicity distribution as a function of radius shows no evidence of significant systematic outward radial migration after this first episode. We argue that the variation of the metallicity dispersion as a function of the guiding radius is dominated by the migration triggered by the bar, but also that the libration of orbits around the bar resonances induces a mixing that may have a significant impact on the observed metallicity dispersion. In contrast, the absence of a break in the metallicity profile of populations younger than about $\sim$6 Gyr and the flattening of the gradient at younger ages is interpreted as evidence that the strength of the bar has decreased, loosening its barrier effect and allowing the gas and metals on both sides of the OLR to mix, erasing the break. Beyond the OLR, stars younger than 7 Gyr show very small metallicity dispersion, suggesting no or limited migration.