Characterisation of local halo building blocks: Thamnos and Sequoia

Abstract: A crucial aspect of galaxy evolution is the pace at which galaxies build up their mass. We investigate this hierarchical assembly by uncovering and timing accretion events experienced by our Galaxy. In the Milky Way, accreted debris has been identified in the local halo, thanks to Gaia. We combine this dataset with advances in colour-magnitude diagram (CMD) fitting to characterise the Galaxy's building blocks based on their age and metallicity distributions. Here, we focus on the retrograde halo, specifically Thamnos and Sequoia. This study, part of the ChronoGal project, uses CMDft.Gaia to fit absolute CMDs of stars from these sub-structures, extracted from a local 5D Gaia DR3 dataset. By comparing their age and metallicity distributions with expected contamination from Gaia Enceladus (GE) and low-energy (LE) in situ populations, we identify distinct stellar population signatures for Sequoia and Thamnos. Both have metal-poor populations ([Fe/H] -2.5 to -1.5 dex) distinct from contamination. Their age distributions reveal the build-up pace of their progenitors: half of Sequoia's stars formed by 12 Gyr ago, while Thamnos appears slightly older and declines faster, forming half its stars by 12.3 Gyr. GE and LE populations formed half their stars by 12.1 Gyr and 12.9 Gyr, respectively. Caution is needed interpreting these distributions, especially for Sequoia, due to small sample sizes that can shift ages younger by up to 1 Gyr. Nonetheless, accounting for this and residual contamination, we conclude Thamnos, Gaia Enceladus, and Sequoia are predominantly old and were accreted within 1-2 Gyr of each other. We present, for the first time, age distributions for the retrograde halo sub-structures Sequoia and Thamnos, derived from photometric data using CMD fitting that also yields metallicity distributions consistent with spectroscopy.