  Using the cosmological zoom simulation VINTERGATAN, we present a new scenario
for the onset of star formation at the metal-poor end of the low-[$\alpha$/Fe]
sequence in a Milky Way-like galaxy. In this scenario, the galaxy is fueled by
two distinct gas flows. One is enriched by outflows from massive galaxies, but
not the other. While the former feeds the inner galactic region, the latter
fuels an outer gas disk, inclined with respect to the main galactic plane, and
with a significantly poorer chemical content. The first passage of the last
major merger galaxy triggers tidal compression in the outer disk, which
increases the gas density and eventually leads to star formation, at a
metallicity 0.75 dex lower than the inner galaxy. This forms the first stars of
the low-[$\alpha$/Fe] sequence. These in situ stars have halo-like kinematics,
similarly to what is observed in the Milky Way, due to the inclination of the
outer disk which eventually aligns with the inner one via gravitational
torques. We show that this tilting disk scenario is likely to be common in
Milky-Way like galaxies. This process implies that the low-[$\alpha$/Fe]
sequence is populated in situ, simultaneously from two formation channels, in
the inner and the outer galaxy, with distinct metallicities. This contrasts
with purely sequential scenarios for the assembly of the Milky Way disk and
could be tested observationally.
