  We study the formation histories and present-day structure of satellite
galaxies formed in a high resolution hydrodynamic simulation of a Milky
Way-like galaxy. The simulated satellites span nearly 4 orders of magnitude in
luminosity but have a very similar mass within their inner 600 pc, ~ 3 10^7
M_solar, with very little scatter. This result is in agreement with the recent
measurements for dwarf spheroidal galaxies (dSphs) in the Milky Way by Strigari
et al. In our simulations a preferred mass scale arises naturally from the
effects of the early reionisation of gas. These impose a sharp threshold of ~
12 km/s on the circular velocity of haloes which can cool gas and make stars.
At the present day, subhaloes that host satellites as luminous as the classical
Milky Way dwarfs (L_V > 2.6 10^5 L_solar), have typically grown to have
circular velocities of $> 20 km/s. There are, however, subhaloes with similar
circular velocities today which were, nevertheless, below threshold at
reionisation and thus remain dark. Star formation in above-threshold haloes is
truncated when the halo is accreted into the main galaxy progenitor. Thus, many
properties of today's dwarf satellites such as their luminosity and star
formation rate are set by their accretion time.
