Constraining SN feedback: a tug of war between reionization and the
  Milky Way satellites
  Theoretical models of galaxy formation based on the cold dark matter
cosmogony typically require strong feedback from supernova (SN) explosions in
order to reproduce the Milky Way satellite galaxy luminosity function and the
faint end of the field galaxy luminosity function. However, too strong a SN
feedback also leads to the universe reionizing too late, and the metallicities
of Milky Way satellites being too low. The combination of these four
observations therefore places tight constraints on SN feedback. We investigate
these constraints using the semi-analytical galaxy formation model galform. We
find that these observations favour a SN feedback model in which the feedback
strength evolves with redshift. We find that, for our best fit model, half of
the ionizing photons are emitted by galaxies with rest-frame far-UV absolute
magnitudes $M_{\rm AB}(1500{\rm \AA})<-17.5$, which implies that already
observed galaxy populations contribute about half of the photons responsible
for reionization. The $z=0$ descendants of these galaxies are mainly galaxies
with stellar mass $M_*>10^{10}\,{\rm M}_{\odot}$ and preferentially inhabit
halos with mass $M_{\rm halo}>10^{13}\,{\rm M}_{\odot}$.
