Early gas stripping as the origin of the darkest galaxies in the Universe

Darkest of the darkDwarf spheroidal galaxies are dominated by dark matter, and within their ranks Draco, Ursa Minor and Andromeda X are 'darkest'. None of the models so far proposed to explain their formation can account for both their exceptional dark matter content and their proximity to a much larger galaxy. Mayer et al. now report simulations showing that their progenitors were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. Tidal shocks and ram pressure stripped all the gas from the progenitors about 10 billion years ago, leaving a tiny stellar component in a more massive dark halo, which is what we see today.