Reignited star formation in dwarf galaxies quenched during reionization
  Irregular dwarf galaxies of the Local Group have very varied properties and
star formation histories. Some of them formed the majority of their stars very
late compared to the others. Extreme examples are Leo A and Aquarius which
reached the peak of star formation at $z<1$ ( > 6 Gyr after BB). This fact
seemingly challenges the LCDM cosmology because the dark matter halos of these
galaxies on average should assemble the majority of their masses before z~2 (<3
Gyr after BB). In this work we investigate whether the delayed star formation
histories of some irregular dwarf galaxies could be explained purely by the
stochasticity of their mass assembly histories coupled with the effect of
cosmic reionization. We develop a semi-analytic model to follow the accretion
of baryonic matter, star formation and stellar feedback in dark matter halos
with present day virial masses 10^9 M_Sun < M < 10^11 M_Sun and with different
stochastic growth histories obtained using the PINOCCHIO code based on
Lagrangian perturbation theory. We obtain the distributions of observable
parameters and the evolution histories for these galaxies. Accretion of
baryonic matter is strongly suppressed after the epoch of reionization in some
models but they continue to accrete dark matter and eventually reach enough
mass for accretion of baryonic matter to begin again. These "reborn" model
galaxies show very similar delayed star formation histories to those of Leo A
and Aquarius. We find that the stochasticity caused by mass assembly histories
is enhanced in systems with virial masses ~10^10 M_Sun because of their
sensitivity to the photoionizing intergalactic radiation field after the epoch
of reionization. This results in qualitatively different star formation
histories in late- and early-forming galaxies and it might explain the peculiar
star formation histories of irregular dwarf galaxies such as Leo A and
Aquarius.
