Over the past 30 years, astronomers have made considerable efforts to observe and analyze the stellar populations of dwarf galaxies. Just over half of these galaxies contain mostly very old stars (6 to 10 billion years old) with very low abundances of elements heavier than Helium. It was deduced that these dwarf galaxies, like the Sculptor’s, had lost their gas in these remote epochs, during which they became satellites orbiting our Galaxy. This scenario had a major cosmological consequence : they had to contain a great deal of dark matter, whose role was to protect their stellar content, which would otherwise have been destroyed by the Milky Way’s gravitational field and accompanying tidal effects.
The discovery of young stars in these dwarf galaxies turns this scenario on its head. Since gas is needed to form stars, this implies that these galaxies had gas between 0.5 and 2 billion years ago. As they accrete into the Milky Way’s halo, gas-rich dwarf galaxies are stripped of their gas, due to the dynamic pressure caused by the hot gas in our Galaxy’s halo. The process is necessarily very rapid, as the mass of dwarf galaxies is considerably less than that of our Galaxy.
The discovery of young stars in spheroidal dwarf galaxies is the result of an in-depth analysis of data from the Gaia satellite, on the one hand, and photometric and spectral analyses carried out at the VLT, on the other. Thanks to Gaia, researchers have literally “filtered out” stars belonging to dwarf galaxies, to exclude those in our Galaxy’s halo. The efficiency of this filtering is unmatched by anything that has gone before. Without Gaia, half the stars observed in the field of view of the Sculptor dwarf galaxy belong to our own Galaxy. With Gaia, this contamination is down to 1.4% !
This has enabled Sculptor’s stars to be clearly seen in the evolutionary phase corresponding to stars aged between 0.5 and 2 billion years, with masses up to 3 solar masses. We know that stars sometimes “rejuvenate” by stealing the mass of a companion, or even merging with it. But to explain stars over 2 solar masses in this way, we’d have to invoke the merger of a triple system, an extremely rare event. The simplest explanation is that these stars are really young, not rejuvenated. The spectra of these stars confirm that their chemical composition is that of the other Sculptor stars. In addition to Sculptor, the dwarf galaxies Sextans, Ursa Minor and Draco also show the presence of a young population, demonstrating that this phenomenon is quite common among dwarf galaxies.
In this case, we must conclude that they have recently lost their gas. This corroborates an alternative scenario for their origins, based on precise measurements of dwarf galaxy orbits obtained by the Gaia satellite. Spheroidal dwarf galaxies have orbital velocities two to three times greater than expected for ancient satellites of the Galaxy, consistent with a recent arrival in the vicinity of the Galaxy, less than three billion years ago. Numerical simulations predict that, in this case, young stars should form in the same (small) proportion as observed. The consequence is considerable, as these models reproduce all the properties of spheroidal dwarf galaxies, but with very little, or even no, dark matter.
CNRS laboratories involved
- Laboratoire Galaxies, étoiles, physique, instrumentation (GEPI - Observatoire de Paris - PSL)
- Tutelles : CNRS / Observatoire de Paris - PSL
- Institut d’astrophysique de Paris (IAP)
- Tutelles : CNRS / Sorbonne University
Reference
Yang et al., The accretion history of the Milky Way IV. Hints of recent star formation in Milky Way dwarf spheroidal galaxies, A&A, 691, A363 (2024).