Rediscovering the Milky Way with an orbit superposition approach and APOGEE data

IV. The disc growth and history of star formation

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³ BIFOLD, Berlin Institute for the Foundations of Learning and Data, Berlin, Germany
⁴ LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 92190 Meudon, France
⁵ Universität Potsdam, Institut für Physik und Astronomie, Karl-Liebknecht-Str. 24-25, 14476 Potsdam, Germany
⁶ Friedrich-Schiller Universität Jena, 07737 Jena, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 September 2025
Accepted: 5 December 2025

Abstract

Context. The Milky Way’s (MW’s) star formation history (SFH) offers insights into the chronology of its assembly and the mechanisms driving its structural development.

Aims. In this study, we present an inference and analysis of the spatially resolved SFH and the MW disc growth.

Methods. Our approach leverages both stellar birth radii estimates and the complete reconstruction of the MW stellar disc using a novel orbit superposition method from APOGEE data, allowing us to trace the orbit-mass weighted SFH based on formation sites, while taking into account stellar mass loss.

Results. We find that the MW is a typical disc galaxy exhibiting inside-out formation: it was compact at z > 2 (R_eff ≈ 2 kpc), had a peak in its star formation rate (SFR) 9–10 Gyr ago, and grew to a present-day size of R_eff ≈ 4.3 kpc. A secondary peak in the SFR ~4 Gyr ago is responsible for the onset of the outer disc, which comprises the metal-poor, low-α population. We find that in situ star formation in the solar neighbourhood started 8–9 Gyr ago. The MW disc is characterised by a negative mean age gradient, as the result of the inside-out growth, with additional flattening induced by stellar radial migration.

Conclusions. Our work showcases the importance of accounting for radial migration and the stellar sample selection function when inferring the SFH and build-up of the MW disc.