The Milky Way is a unique laboratory , where stellar properties can be measured and analyzed in detail .
In particular , stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy .
In this article , we analyze the kinematics , spatial distribution , and chemistry of a large number of stars in the Solar Neighborhood , where all of the main Galactic components are well-represented .
We find that the thick disk comprises two distinct and overlapping stellar populations , with different kinematic properties and chemical compositions .
The metal-weak thick disk ( MWTD ) contains two times less metal content than the canonical thick disk , and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy .
The rotational velocity of the MWTD around the Galactic center is \sim 150 km s ^ { -1 } , corresponding to a rotational lag of 30 km s ^ { -1 } relative to the canonical thick disk ( \sim 180 km s ^ { -1 } ) , with a velocity dispersion of 60 km s ^ { -1 } .
This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy ’ s assembly , or it is a precursor disk , formed in the inner Galaxy and brought into the Solar Neighborhood by bar instability or spiral-arm formation mechanisms .