A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages
7 affiliations
- 1Instituto de Astrofísica de Canarias
- 2Universidad de La Laguna
- 3Universidad de Granada
- 4INAF–Astronomical Observatory of Abruzzo
- 5INFN
- 6University of Victoria
- 7Korea University
Abstract
The spatial distribution of Galactic disk stars of different ages and metallicities encodes its dynamical evolution history and formation pathways. The lack of precise ages for large, representative samples of stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk (e.g. upside-down formation versus secular dynamical heating). In this letter, we use color–magnitude diagram fitting of Gaia data to derive very precise age–metallicity distributions for complete stellar samples in the solar neighborhood. We identify quasi-mono-age–metallicity (qMAM) stellar populations with an age resolution better than 5% (≃0.5 Gyr at old age), and study their absolute vertical density profile. We observe a clear correlation between scale height and stellar age, with younger populations consistently more confined to the plane. We find evidence that the scale height decrease rate changed drastically 10.8 5 − 0.12 + 0.13 Gyr ago: at older ages, the evolution is very steep, with h Z dropping from over 1.26 to ∼0.5 kpc in merely 2 Gyr; subsequently, the thinning is very gradual, with h Z ∼ 0.1 kpc for the youngest populations. The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed. We thus strongly support an evolutionary scenario defined by two distinct temporal phases: an initial, highly turbulent epoch that gave birth to the thick disk, and a subsequent more quiescent period of secular evolution and gradual gas settling, which formed the thin disk.
Versions
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BibTeX
@article{mirabal2026twostage,
title = {{A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages}},
author = {David Mirabal and Carme Gallart and Tomás Ruiz-Lara and Emma Fernández-Alvar and Anna B. A. Queiroz and Guillem Aznar-Menargues and Chris B. A. Brook and Santi Cassisi and Yllari K. González-Koda and Julio F. Navarro and Pedro A. Palicio and Francisco Surot},
journal = {The Astrophysical Journal Letters},
year = {2026},
month = oct,
doi = {10.3847/2041-8213/aeb0a3},
url = {https://iopscience.iop.org/article/10.3847/2041-8213/aeb0a3},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A Two-stage Settling of the Milky Way Disk Revealed by Precise ChronoGal Ages
AU - David Mirabal
AU - Carme Gallart
AU - Tomás Ruiz-Lara
AU - Emma Fernández-Alvar
AU - Anna B. A. Queiroz
AU - Guillem Aznar-Menargues
AU - Chris B. A. Brook
AU - Santi Cassisi
AU - Yllari K. González-Koda
AU - Julio F. Navarro
AU - Pedro A. Palicio
AU - Francisco Surot
PY - 2026
DA - 2026/10/16
JO - The Astrophysical Journal Letters
DO - 10.3847/2041-8213/aeb0a3
UR - https://iopscience.iop.org/article/10.3847/2041-8213/aeb0a3
AB - The spatial distribution of Galactic disk stars of different ages and metallicities encodes its dynamical evolution history and formation pathways. The lack of precise ages for large, representative samples of stars has traditionally been a drawback for securely differentiating among formation scenarios proposed to explain the vertical properties of the Milky Way disk (e.g. upside-down formation versus secular dynamical heating). In this letter, we use color–magnitude diagram fitting of Gaia data to derive very precise age–metallicity distributions for complete stellar samples in the solar neighborhood. We identify quasi-mono-age–metallicity (qMAM) stellar populations with an age resolution better than 5% (≃0.5 Gyr at old age), and study their absolute vertical density profile. We observe a clear correlation between scale height and stellar age, with younger populations consistently more confined to the plane. We find evidence that the scale height decrease rate changed drastically 10.8 5 − 0.12 + 0.13 Gyr ago: at older ages, the evolution is very steep, with h Z dropping from over 1.26 to ∼0.5 kpc in merely 2 Gyr; subsequently, the thinning is very gradual, with h Z ∼ 0.1 kpc for the youngest populations. The extremely rapid early vertical settling of the disk points to a fundamentally upside-down formation process at early times, when the thick disk component is basically formed. We thus strongly support an evolutionary scenario defined by two distinct temporal phases: an initial, highly turbulent epoch that gave birth to the thick disk, and a subsequent more quiescent period of secular evolution and gradual gas settling, which formed the thin disk.
SN - 2041-8205
SN - 2041-8213
ER -