Dark Satellites of the Milky Way

We study the formation histories and present-day structure of galactic subsystems formed in a high resolution hydrodynamic simulation of a Milky Way-sized galaxy. The simulated satellites span nearly 4 orders of magnitude in luminosity but have a very similar mass within their inner 600 pc with very little scatter. This is in agreement with the recent measurements for dwarf spheroidal galaxies (dSphs). Interestingly, subhaloes with circular velocity around 20 km/s sometimes host satellites that are as bright as the classical satellites and sometimes they are found as completely dark objects. We find that there is a critical circular velocity at the epoch of reionization, below which haloes fail to host star formation and found as "dark satellites" today.