On the origin of the chemical bimodality of disk stars: A tale of merger
  and migration
  The Milky Way's stellar disk exhibits a bimodality in the [Fe/H] vs.
[$\alpha$/Fe] plane, showing a distinct high-$\alpha$ and low-$\alpha$ sequence
whose origin is still under debate. We examine the [Fe/H]-[$\alpha$/Fe]
abundance plane in cosmological hydrodynamical simulations of Milky Way like
galaxies from the NIHAO-UHD project and show that the bimodal $\alpha$-sequence
is a generic consequence of a gas-rich merger at some time in the Galaxy's
evolution. The high-$\alpha$ sequence evolves first in the early galaxies,
extending to high metallicities, while it is the low-$\alpha$ sequence that is
formed after the gas-rich merger. The merger brings in fresh metal-poor gas
diluting the interstellar medium's metallicity while keeping the [$\alpha$/Fe]
abundance almost unchanged. The kinematic, structural and spatial properties of
the bimodal $\alpha$-sequence in our simulations reproduces that of
observations. In all simulations, the high-$\alpha$ disk is old, radially
concentrated towards the galaxy's center and shows large scale heights. In
contrast, the low-$\alpha$ disk is younger, more radially extended and
concentrated to the disk mid-plane. Our results show that the abundance plane
is well described by these two populations that have been distributed radially
across the disk by migration: at present-day in the solar neighbourhood,
low-$\alpha$ stars originate from both the inner and outer disk while most of
the high-$\alpha$ stars have migrated from the inner disk. We show that age
dating the stars in the [Fe/H]-[$\alpha$/Fe] plane can constrain the time of
the low-$\alpha$ sequence forming merger and conclude that $\alpha$-bimodality
is likely a not uncommon feature of disk galaxies.
