Studies of the ages , abundances , and motions of individual stars in the Milky Way provide one of the best ways to study the evolution of disk galaxies over cosmic time .
The formation of the Milky Way ’ s barred inner region in particular is a crucial piece of the puzzle of disk galaxy evolution .
Using data from APOGEE and Gaia , we present maps of the kinematics , elemental abundances , and age of the Milky Way bulge and disk that show the barred structure of the inner Milky Way in unprecedented detail .
The kinematic maps allow a direct , purely kinematic determination of the bar ’ s pattern speed of 41 \pm 3 \mathrm { km s } ^ { -1 } \mathrm { kpc } ^ { -1 } and of its shape and radial profile .
We find the bar ’ s age , metallicity , and abundance ratios to be the same as those of the oldest stars in the disk that are formed in its turbulent beginnings , while stars in the bulge outside of the bar are younger and more metal-rich .
This implies that the bar likely formed \approx 8 \mathrm { Gyr } ago , when the decrease in turbulence in the gas disk allowed a thin disk to form that quickly became bar-unstable .
The bar ’ s formation therefore stands as a crucial epoch in the evolution of the Milky Way , a picture that is in line with the evolutionary path that emerges from observations of the gas kinematics in external disk galaxies over the last \approx 10 \mathrm { Gyr } .