Characterisation of local halo building blocks: Thamnos and Sequoia

Context . A crucial aspect of galaxy evolution is the pace at which galaxies build up their mass. We can investigate this hierarchical assembly by uncovering and timing accretion events experienced by our Galaxy. Aims . In the Milky Way, accreted debris has been previously identified in the local halo, thanks to the advent of Gaia data. We aim to couple this dataset with advancements in colour–magnitude diagram (CMD) fitting techniques to characterise the building blocks of the Galaxy, based on their age and metallicity distributions. Here, we focus on the retrograde halo, specifically those of Thamnos and Sequoia. Methods . We conducted this study as part of the ChronoGal project by fitting the absolute CMDs (using CMDft. Gaia ) of samples of stars associated with these sub-structures, extracted from a local 5D Gaia DR3 dataset. Comparing their derived age and metallicity distributions with those of the expected contamination, from the dominant Gaia Enceladus (GE) and low-energy (LE) in situ populations, we can unveil the stellar population signatures of the progenitors of Sequoia and Thamnos. Results . We show that both Thamnos and Sequoia have a metal-poor population ([Fe/H] ~−2.5 to −1.5 dex) that is distinct from the expected contamination. Their age distributions offer us the ability to see the pace of the build-up of their progenitors. Half of the stars in Sequoia were formed by the look-back time of ~12 −0.3 +0.3 Gyr. Thamnos appears slightly older, on average, and declines quickly, having formed half its stars at ~12.3 −0.3 +0.3 Gyr. Compared to GE and the LE in situ populations, they formed half of their stars by 12.1 −0.1 +0.1 Gyr and 12.9 −0.1 +0.1 Gyr, respectively. Caution should be taken when interpreting the age distributions, especially that of Sequoia, due to the low number of stars, which can cause shifts to younger ages of up to ~1 Gyr. However, considering these potential shifts and the underlying contamination that is inherently difficult to remove completely, our results allow us to safely conclude that Thamnos, Gaia Enceladus, and Sequoia are all predominantly old and accreted at similar epochs, within ~1–2 Gyr of each other. Conclusions . We present, for the first time, the age distributions of the retrograde halo sub-structures: Sequoia and Thamnos. These have been derived from purely photometric data using CMD fitting techniques, which also provide metallicity distributions that successfully reproduce the results from spectroscopy, highlighting the applicability of CMDft. Gaia .