Cold accretion flows and chemical bimodality of the Milky Way galaxy
  Abundance of chemical elements in the stars provides important clues
regarding galaxy formation. The most powerful diagnostics is the relative
abundance of {\alpha}-elements (O, Mg, Si, S, Ca, and Ti) with respect to iron
(Fe), [{\alpha}/Fe], each of which is produced by different kinds of
supernovae. The existence of two distinct groups of stars in the solar
neighbourhood, one with high [{\alpha}/Fe] and another with low [{\alpha}/Fe],
suggests that the stars in the solar vicinity have two different origins.
However, the specific mechanism of the realization of this bimodality is
unknown. Here, we show that the cold flow hypothesis recently proposed for the
accretion process of primordial gas onto forming galaxies predicts two episodes
of star formation separated by a hiatus 6-7 Gyr ago and naturally explains the
observed chemical bimodality. We found that the first phase of star formation
that forms high [{\alpha}/Fe] stars is caused by the 'genuine' cold flow, in
which unheated primordial gas accretes to the galactic disk in a freefall
fashion. The second episode of star formation that forms low [{\alpha}/Fe]
stars is sustained by much slower gas accretion as the once-heated gas
gradually cools by radiation. The cold flow hypothesis can also explain the
large-scale variation in the abundance pattern observed in the Milky Way galaxy
in terms of the spatial variation of gas accretion history.
