Young Stars Discovered in Dwarf Spheroidal Galaxies Confirm their Recent Infall into the Milky Way

Recent observations from ESA’s Gaia satellite and with ESO’s Very Large Telescope have identified the presence of a population of young stars, 0.5 to 2 Gyr old, in the halo of, and in dwarf spheroidal galaxies surrounding, the Milky Way (MW). It suggests that MW dwarf galaxies, currently devoid of gas, had, until recent times, enough gas to sustain a burst of star formation. The recent loss of gas coincides with their arrival in the vicinity of the MW, in agreement with orbital predictions from Gaia that indicate that most dwarf galaxies reached the MW halo less than 3 Gyr years ago. This completely changes the interpretation of their dynamics, mass and dark matter content.