Astronomers in Chile reveal the origin of the Small Magellanic Cloud

A study focusing on star clusters revealed an ancient merger recorded 6 billion years ago, which would have given rise to this galaxy.

The history of the Small Magellanic Cloud (SMC), one of the galaxies closest to the Milky Way, has just taken on a new level of complexity. The study “The VISCACHA survey XIV Chemical evolution history of the SMC: The Southern Bridge Clusters,” led by Saroon Sasi, Researcher at the Center for Astrophysics and Related Technologies (CATA), and Bruno Dias, Associate Researcher at CATA and professor at UNAB, revealed that the star clusters in the Southern Bridge (SB) region share the same chemical signature as those in the West Halo (WH).

The research also involves the participation of CATA Principal Investigator and UNAB academic Dante Minniti. This finding confirms that the Small Magellanic Cloud was formed from a large galactic merger that occurred 6 billion years ago, an event that redefined its structure and chemical composition.

Star clusters are ideal “fossils” of galactic evolution, as they preserve the chemical and physical conditions of the environments in which they were born. Unlike field stars, which can migrate and mix over time, these are relatively closed systems. Their stellar populations are contemporaneous and chemically homogeneous, making them accurate tracers of the interstellar medium at specific times,” says Saroon Sasi, also a doctoral student at UNAB.

In dynamically complex regions such as Southern Bridge (SB) and West Halo (WH), where tidal interactions, mergers, and gas flows have sculpted the outskirts of the Small Magellanic Cloud, star clusters serve as time capsules. Their age-metallicity ratio (AMR), elemental abundances, and spatial distributions encode the traces of past events. Through them, we can reconstruct the evolutionary timeline of the galaxy with greater fidelity,” explains Sasi.

The team analyzed 67% of the known clusters in the Southern Bridge, applying the same methodology previously used in the West Halo region. The result was revealing: both areas show a virtually identical pattern of chemical evolution, characterized by a sharp drop in metallicity about 6 billion years ago, followed by a period of accelerated enrichment, giving rise to the Small Magellanic Cloud as we know it today.

“The only way to explain this sharp decline is that the galaxy received a large injection of gas very poor in metals from another galaxy. It was not a minor event, but a large-scale merger that left a chemical footprint that we can still measure today,” says Bruno Dias.

“To gather this strong evidence, it was necessary to analyze the population of star clusters that serve as fossils of the chemical evolution of the Small Magellanic Cloud using high-quality data observed by the VISCACHA project and analyzed in a uniform manner. In this way, we are able to accurately measure their age, chemical composition, distances, and position in space,” explains the CATA Adjunt Researcher.

Previous history of galaxies

The Small Magellanic Cloud (SMC) is an irregular dwarf galaxy with complex characteristics in terms of its structure, dynamics, and chemical evolution. However, the work being carried out and the results of previous studies by the VISCACHA project show that its history is much richer and more dynamic.

Our findings show that this merger introduced pristine, metal-poor gas into the SMC, causing a sharp drop in metallicity followed by a burst of star formation. This chemical signature is preserved in star clusters in the Southern Bridge and West Halo regions. What makes this discovery particularly interesting is that it suggests that the SMC is not just a small, irregular galaxy, but is the result of a more complex formation process. The merger not only shaped its outskirts, but probably forged the SMC’s current structure and chemical identity,” explains Sasi.

The CATA Research Fellow also explains that this is a fundamental event for understanding how dwarf galaxies evolve. “Similar discoveries have been made in larger systems. For example, the Milky Way’s halo was formed in part through a large merger with the Gaia-Sausage-Enceladus galaxy about 8-10 billion years ago. Other galaxies, such as M64, show signs of minor mergers and counter-rotating gas disks. Unlike those distant or massive systems, the SMC is close by and significantly poorer in metals.”

Next steps: broadening our perspective

The team now seeks to extend the research to other poorly explored areas of the SMC, such as the Counter Bridge and Northern Bridge regions, to determine whether they also bear the mark of that great galactic merger or whether they followed different enrichment trajectories.

“We will conduct high-resolution spectroscopic studies of key clusters in these regions, searching for finer chemical traces. This will allow us to pinpoint the timing, nature, and impact of mergers in the complete history of the galaxy,” says the doctoral student.

This study, and previous related studies, are part of the VISCACHA Survey, an international project that seeks to understand the chemical and structural evolution of the galaxies closest to ours. The project is led by Bruno Dias, a CATA researcher and UNAB academic. For CATA, these results consolidate its contribution to cutting-edge research on galactic formation.

“Each cluster we study gives us pieces of a puzzle that reveals how the first galaxies formed. The fact that we can do this from Chile, contributing to global astronomy, is a source of pride and an incentive to continue researching,” concludes CATA researcher Bruno Dias.