The edge of galaxy formation I: formation and evolution of MW-satellites
  analogues before accretion
  The satellites of the Milky Way and Andromeda represent the smallest galaxies
we can observe in our Universe. In this series of papers we aim to shed light
on their formation and evolution using cosmological hydrodynamical simulations.
In this first paper we focus on the galaxy properties before accretion, by
simulating twenty seven haloes with masses between $5\times 10^8$ and $10^{10}
M_\odot$. Out of this set nineteen haloes successfully form stars, while eight
remain dark. The simulated galaxies match quite well present day observed
scaling relations between stellar mass, size and metallicity, showing that such
relations are in place before accretion. Our galaxies show a large variety of
star formation histories, from extended star formation periods to single
bursts. As in more massive galaxies, large star formation bursts are connected
with major mergers events, which greatly contribute to the overall stellar mass
build up. The intrinsic stochasticity of mergers induces a large scatter in the
stellar mass halo mass relation, up to two orders of magnitude. Despite the
bursty star formation history, on these mass scales baryons are very
ineffective in modifying the dark matter profiles, and galaxies with a stellar
mass below $\approx 10^6 M_\odot$ retain their cuspy central dark matter
distribution, very similar to results from pure N-body simulations.
