The VISCACHA survey

XIV. Chemical evolution history of the SMC: The Southern Bridge Clusters

¹ Instituto de Astrofísica, Departamento de Física y Astronomía, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha, 700, Las Condes, Santiago, Chile
² National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
³ Instituto de Física – Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 149, Rio de Janeiro 21941-909, Brazil
⁴ Departamento de Física, ICEx – Universidade Federal de Minas Gerais, Av. Antônio Carlos 6627, Belo Horizonte 31270-901, Brazil
⁵ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁶ Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
⁷ Instituto de Astronomía Teórica y Experimental (CONICET-UNC), Laprida 854, X5000BGR Córdoba, Argentina
⁸ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 106, C. P. 22800 Ensenada, B. C., Mexico
⁹ Vatican Observatory, V00120 Vatican City State, Italy
¹⁰ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina
¹¹ Instituto de Astrofísica de La Plata (CCT La Plata, UNLP-CONICET), Paseo del Bosque s/n, 1900 La Plata, Argentina
¹² Universidade FEderal Do Rio Grande do Sul, DEpartamento de AStronomia CP15051, Port Alegre 91501-970, Brazil
¹³ Universidade Federal do ABC, Centro de Ciências Naturais e Humanas, Avenida dos Estados, 5001, 09210-580, Brazil
¹⁴ Universidade Estadual de Santa Cruz (UESC), Departamento de Ciências Exatas, Rodovia Jorge Amado km 16, 45662-900 Ilhéus, Brazil
¹⁵ Departamento de Astronomia, Casilla 160-C, Universidad de Concepcion, Chile
¹⁶ Departamento de Astronomía, Facultad de Ciencias, Universidad de La Serena., Av.Raul Bitran 1305, La Serena, Chile
¹⁷ NSF NOIRLab/NSF–DOE Vera C. Rubin Observatory HQ, 950 N. Cherry Ave., Tucson, AZ 85719, USA (AURA Staff)
¹⁸ Laboratório Nacional de Astrofísica LNA/MCTI, 37504-364 Itajubá, MG, Brazil
¹⁹ ESO – European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
²⁰ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile

^⋆ Corresponding author: saroonsasi19@gmail.com

Received: 7 May 2025
Accepted: 16 June 2025

Abstract

The chemical evolution history of the Small Magellanic Cloud (SMC) is complex and is best understood through a comprehensive analysis of star clusters across its various regions. The VISCACHA survey aims to fully explain the chemical evolution of SMC star clusters by analyzing different sub-regions adopted from an existing framework. The west halo (WH) region, which contains the oldest and most metal-poor stellar populations, exhibits a clear age-metallicity relation (AMR) with minimal dispersion. This region shows a significant dip of ∼0.5 dex in metallicity approximately 6 Gyr ago. This was likely caused by a major merger event that subsequently accelerated the star formation rate. Clusters in the Southern Bridge (SB) and Northern Bridge regions of the SMC may have experienced distinct chemical enrichment histories, as suggested by our previous works but with limited data coverage. Furthermore, the AMR of wing/bridge (W/B) shows no sign of enrichment caused by the aforementioned merger event, but exhibits signatures of the recent collisions between the clouds contemporaneous with the epochs of the Magellanic Stream and Bridge formations. In this study, we present an updated AMR for the SB region based on a sample that includes approximately 67% of its known clusters. Contrary to the expectation of a very unique chemical evolution history, these SB clusters show a trend similar to the one of the WH clusters. The chemical evolution models that best fit the AMR trend of the SB clusters show excellent agreement with the major merger model proposed for the WH clusters. Building on this, we suggest a new unified chemical evolution model for both the WH and SB clusters, which can be explained by a major merger at ∼6 Gyr followed by episodic chemical enrichment over time.