The accretion history of the Milky Way IV. Hints of recent star formation in Milky Way dwarf spheroidal galaxies

TL;DR

This study reports the discovery of a young stellar population in Milky Way dwarf spheroidal galaxies, suggesting recent star formation likely triggered by recent infall into the MW halo, challenging previous assumptions of exclusively old populations.

Contribution

It provides evidence for recent star formation in dwarf spheroidal galaxies, linked to their recent infall into the Milky Way, based on Gaia data and stellar population analysis.

Findings

Detection of possibly young stars (~1 Gyr old) in dwarf spheroidals.

Evidence against blue straggler explanation for these stars.

Consistent with recent infall and star formation triggered by ram pressure.

Abstract

Dwarf spheroidal galaxies are known to be dominated by old stellar populations. This has led to the assumption that their gas-rich progenitors lost their gas during their infall in the Milky Way (MW) halo at distant look-back times. Here, we report a discovery of a tiny but robustly detected population of possibly young ($\sim$ 1 Gyr old) and intermediate-mass ($\rm 1.8 M_{\odot} \le M < 3 M_{\odot}$) stars in MW dwarf spheroidal galaxies. This was established on the basis of their positions in color-magnitude diagrams, after filtering out the bulk of the foreground MW using Gaia DR3 proper motions. We have considered the possibility that this population is made of evolved blue stragglers. For Sculptor, it seems unlikely, because 95.5% of its stars are older than 8 Gyr, leading to masses smaller than 0.9 M$_{\odot}$. This would only allow blue straggler masses of less than 1.8 M$_{\odot}$, which is much lower than what we observed. Alternatively, it would require the merger of three turnoff stars, which appears even more unlikely. On the other hand, the recent Gaia proper motion measurements of MW dwarf galaxies infer their low binding energies and large angular momenta, pointing to a more recent, $\le$ 3 Gyr, infall. Although the nature of the newly discovered stars still needs further confirmation, we find that they are consistent with the recent infall of the dwarf galaxies into the MW halo, when star formation occurred from the ram pressurization of their gas content before its removal by the hot Galactic corona. The abundance of this plausibly young population of stars is similar to the expectations drawn from hydrodynamical simulations. These results point to a novel origin for MW dwarf spheroidal galaxies.