The Galactic inner spiral arms revealed by the Gaia ESO Survey chemical abundances

Metallicity and [Mg/Fe] ratios

¹ Institute of Theoretical Physics and Astronomy, Vilnius University, Sauletekio av. 3, 10257 Vilnius, Lithuania
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy
³ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁴ IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Trieste, Italy
⁵ Dipartimento di Fisica, Sezione di Astronomia, Università di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁶ INFN, Sezione di Trieste, Via A. Valerio 2, 34127 Trieste, Italy
⁷ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy

^★ Corresponding author: carlos.viscasillas@ff.vu.lt

Received: 21 February 2025
Accepted: 23 April 2025

Abstract

Context. The spiral structure of the Milky Way has traditionally been mapped using stellar density, kinematics, and gas distribution. However, chemical abundances – especially in the inner Galaxy – remain a relatively under-explored tracer, offering valuable insights into its formation and evolution. Recent observational advances, such as Gaia DR3 GSP-Spec, have highlighted the potential of chemical abundances in tracing and revealing the structure of spiral arms.

Aims. Building on these studies, we aim to trace the Milky Way’s inner spiral arms using chemical abundance data from the Gaia-ESO Survey. By mapping over-densities in [Fe/H] and [Mg/Fe], we seek to identify spiral arms in both radial and vertical planes, detect substructures, and compare our results with recent Galactic chemical evolution models.

Methods. We used chemical abundance data from the Gaia-ESO Survey to create spatial maps of [Fe/H], [Mg/H], and [Mg/Fe] excess across the Galactic inner disc. The maps were analysed to detect over-densities associated with known spiral arms. We compared our results with recent spiral arm models.

Results. For the first time, the inner spiral arms have been revealed using chemical abundance patterns. We detect [Fe/H] enhancements and [Mg/Fe] under-abundances that consistently trace the Scutum and Sagittarius arms. A spur connecting these arms is observed in the [Mg/H] plane. The alignment between our observations and the results of our 2D chemical evolution models reinforces the significance of spiral arm transits in driving both azimuthal and radial variations in chemical abundances.

Conclusions. Our results confirm that spiral arms can be traced using stellar chemical abundances with Gaia-ESO data, providing a new perspective on the structure of the inner Galaxy. The consistency between higher [Fe/H] and lower [Mg/Fe] ratios, as observed in previous studies, further supports our findings. The observed spur, bifurcation, and vertical substructures align well with recent models and studies, indicating that chemical maps can significantly contribute to our understanding of Galactic spiral arms.