New Insights into the Milky Way's Halo: Thamnos and Sequoia Characterised

Recent research has focused on the characterisation of two significant components of the Milky Way's halo, known as Thamnos and Sequoia. This study, conducted by Emma Dodd and colleagues, utilizes data from the Gaia mission to explore the formation and evolution of these halo structures. The findings indicate that both Thamnos and Sequoia exhibit a metal-poor stellar population, with metallicity levels below -1.5 dex, distinguishing them from other populations in the halo.

The research highlights that Sequoia formed approximately half of its stars around 12 billion years ago, while Thamnos is slightly older, with half of its stars formed at around 12.3 billion years ago. This timeline suggests that Thamnos was accreted into the Milky Way earlier than Sequoia, which is identified as the most recent accretion event. The study presents the first age distributions of these retrograde halo substructures, derived through advanced photometric techniques, which also align closely with spectroscopic data.

This research contributes to our understanding of galaxy formation and evolution, providing insights into the hierarchical assembly of the Milky Way. By identifying the distinct characteristics of Thamnos and Sequoia, the study enhances our knowledge of the processes that shaped our galaxy's structure and the history of its stellar populations. The full paper can be accessed through arXiv with the identifier arXiv:2408.13763.