STEP survey

III. STEPping stones between the clouds: the star formation history of the Magellanic Bridge^★

¹ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
² Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
³ Dipartimento di Fisica, Università di Pisa, Largo Bruno Pontecorvo 3, 56127, Pisa, Italy
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Piero Gobetti 93/3, 40129 Bologna, Italy
⁵ INFN, Largo B. Pontecorvo 3, 56127 Pisa, Italy
⁶ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße. 12-14, 69120 Heidelberg, Germany
⁷ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁸ INFN – Gruppo Collegato di Salerno-Sezione di Napoli, Dipartimento di Fisica “E.R. Caianiello”, Università di Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy

^★★ Corresponding author.

Received: 2 September 2025
Accepted: 11 February 2026

Abstract

Context. The Magellanic Clouds (MCs) offer a unique laboratory for studying galaxy interaction and the evolution of dwarf galaxies. The star formation history (SFH), which traces when and how stars formed, provides powerful constraints for the dynamical modelling of the system’s past interactions and the processes of stripping and triggered star formation in tidally influenced environments.

Aims. We aim to reconstruct the SFH of the Magellanic Bridge, the gaseous and stellar stream connecting the Magellanic Clouds. We used data from the deep optical STEP survey, which covers 54 deg² across the Small Magellanic Cloud (SMC) and the Bridge, reaching stars below the oldest main-sequence turnoff at the distance of the MCs.

Methods. We applied the synthetic colour-magnitude diagram technique to 14 deg² of STEP data. We constructed two libraries of synthetic stellar populations based on the PARSEC-COLIBRI and BaSTI stellar evolutionary models, with metallicities in the range −2.0 ≤ [Fe/H] ≤ 0 across the whole Hubble time.

Results. We find a clear peak of recent star formation ∼100 Myr ago in the Magellanic Bridge, which becomes increasingly pronounced towards the SMC. The low metallicity of this population suggests that it formed from gas stripped from the SMC during its most recent close encounter with the LMC. In the eastern part of the Bridge (LMC side), star formation peaks at earlier times, around 10 Gyr and 2 Gyr ago. We estimate a total stellar mass in the Bridge of (5.1 ± 0.2) × 10⁵M_⊙ and a present-day stellar metallicity of [Fe/H]~-0.6 dex, close to SMC value.