A radial age gradient in the geometrically thick disk of the Milky Way
  In the Milky Way, the thick disk can be defined using individual stellar
abundances, kinematics, or age; or geometrically, as stars high above the
mid-plane. In nearby galaxies, where only a geometric definition can be used,
thick disks appear to have large radial scale-lengths, and their red colors
suggest that they are uniformly old. The Milky Way's geometrically thick disk
is also radially extended, but it is far from chemically uniform:
alpha-enhanced stars are confined within the inner Galaxy. In simulated
galaxies, where old stars are centrally concentrated, geometrically thick disks
are radially extended, too. Younger stellar populations flare in the simulated
disks' outer regions, bringing those stars high above the mid-plane. The
resulting geometrically thick disks therefore show a radial age gradient, from
old in their central regions to younger in their outskirts. Based on our age
estimates for a large sample of giant stars in the APOGEE survey, we can now
test this scenario for the Milky Way. We find that the geometrically-defined
thick disk in the Milky Way has indeed a strong radial age gradient: the median
age for red clump stars goes from ~9 Gyr in the inner disk to 5 Gyr in the
outer disk. We propose that at least some nearby galaxies could also have thick
disks that are not uniformly old, and that geometrically thick disks might be
complex structures resulting from different formation mechanisms in their inner
and outer parts.
