Unveiling the past evolution of the progenitor of the Helmi streams

Abstract

We aim to determine unique features that characterise the past evolution of the progenitor of the Helmi streams through the analysis of star formation histories (SFHs). From the 5D {\it Gaia} EDR3 dataset, we extracted local samples of stars dominated by the Helmi streams, the Galactic (thick and thin) disc, and the local retrograde halo. We did this by identifying regions in a pseudo-Cartesian velocity space (obtained by setting line-of-sight velocities to zero), where stars belonging to these components, as identified in samples with 6D phase-space information, are predominantly found. We made use of an updated absolute colour-magnitude diagram (CMD) fitting methodology to contrast the SFHs of these samples to unveil distinct signatures of the past evolution of a disrupted accreted system: the Helmi streams. To this end, special attention was given to the correct characterisation of {\it Gaia} completeness effects and observational errors on the CMD. We further investigated alternative sample selections to improve the purity of our 5D Helmi stream catalogues. We find that the progenitor of the Helmi streams experienced an early star formation that was sustained for longer (until 7--9~Gyr ago) than for the Milky Way halo (10--11~Gyr ago). As a consequence, half of its stellar mass was in place $\sim$~0.7~Gyr later. The quenching of star formation in the Helmi stream progenitor $\sim$~8~Gyr ago suggests it was accreted by the Milky Way around this time, in concert with previous estimates based on the dynamics of the streams.

6 pages (+1 in appendix), 4 figures (+1 in appendix), accepted for publication in A&A Letters (version after language edition)

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