Chronology of our galaxy from Gaia colour-magnitude diagram fitting (ChronoGal)

Ruiz-Lara, Tomás, Mirabal, David, Gallart, Carme, Grand, Robert, Fragkoudi, Francesca, Pérez, Isabel, Cassisi, Santi, Fernández-Alvar, Emma, Queiroz, Anna B., Aznar-Menargues, Guillem, González-Koda, Yllari K., Rivero, Alicia, Surot, Francisco, Thomas, Guillaume F., Bieri, Rebekka, Gomez, Facundo A., Pakmor, Rüdiger and van de Voort, Freeke ORCID: https://orcid.org/0000-0002-6301-638X 2026. Chronology of our galaxy from Gaia colour-magnitude diagram fitting (ChronoGal). Astronomy & Astrophysics 705 , A92. 10.1051/0004-6361/202556468

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Abstract

The Milky Way’s inner region is dominated by a stellar bar and a boxy-peanut-shaped bulge. However, which stellar populations inhabit the inner Galaxy or how star formation proceeded there is still unknown. The difficulty in studying these stars stems from their location in dense regions that are strongly impacted by extinction and crowding effects. In this work we used star formation histories computed in the solar neighbourhood via Gaia colour-magnitude diagram fitting to shed light on the evolution of the central regions of our Galaxy. For that, we obtained precise age distributions for the non-negligible amount of super-metal-rich stars ([M/H] ∼ 0.5) in the solar neighbourhood (more than 5% of the total stars within 400 pc of the plane). Assuming that these stars were born in the inner Galaxy and migrated outwards, those distributions should be indicative of the true stellar age distribution in the inner Galaxy. Surprisingly, we find that these age distributions are not continuous but show clear signs of episodic star formation (∼13.5, 10.0, 7.0, 4.0, 2.0, and less than 1 Gyr ago). Interestingly, with the exception of the 4 Gyr event, the timings of the detected events coincide with the formation of the primitive Milky Way and with known merging events or satellite encounters (Gaia-Enceladus-Sausage, Sagittarius dwarf galaxy, and the Magellanic Clouds), suggesting that these events could have triggered global star-forming episodes. These results are compatible with a scenario in which Gaia-Enceladus-Sausage is responsible for the formation of the bar 10 Gyr ago. However, we cannot associate any accretion counterpart with the event that occurred 4 Gyr ago, leaving open the possibility of a late formation of the bar, as previously proposed. The Auriga Superstars simulations also indicate that metal-rich stars in the solar neighbourhood-like regions formed at discrete times and migrated from the inner parts of barred galaxies, suggesting a possible link to bar dynamics and satellite accretion. This novel analysis allows us to indirectly witness the evolution of the inner Milky Way and constrain dynamical models of the Milky Way bar.

Item Type:	Article
Date Type:	Publication
Status:	Published
Schools:	Schools > Physics and Astronomy
Additional Information:	License information from Publisher: LICENSE 1: URL: https://creativecommons.org/licenses/by/4.0, Start Date: 2026-01-09
Publisher:	EDP Sciences
ISSN:	0004-6361
Date of First Compliant Deposit:	22 January 2026
Date of Acceptance:	26 September 2025
Last Modified:	22 Jan 2026 10:01
URI:	https://orca.cardiff.ac.uk/id/eprint/184094

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