  We study the structure, age and metallicity gradients, and dynamical
evolution using a cosmological zoom-in simulation of a Milky Way-mass galaxy
from the Feedback in Realistic Environments project. In the simulation, stars
older than 6 Gyr were formed in a chaotic, bursty mode and have the largest
vertical scale heights (1.5-2.5 kpc) by z=0, while stars younger than 6 Gyr
were formed in a relatively calm, stable disk. The vertical scale height
increases with stellar age at all radii, because (1) stars that formed earlier
were thicker "at birth", and (2) stars were kinematically heated to an even
thicker distribution after formation. Stars of the same age are thicker in the
outer disk than in the inner disk (flaring). These lead to positive vertical
age gradients and negative radial age gradients. The radial metallicity
gradient is neg- ative at the mid-plane, flattens at larger disk height |Z|,
and turns positive above |Z|~1.5kpc. The vertical metallicity gradient is
negative at all radii, but is steeper at smaller radii. These trends broadly
agree with observations in the Milky Way and can be naturally understood from
the age gradients. The vertical stellar density profile can be well-described
by two components, with scale heights 200-500 pc and 1-1.5 kpc, respectively.
The thick component is a mix of stars older than 4 Gyr which formed through a
combination of several mechanisms. Our results also demonstrate that it is
possible to form a thin disk in cosmological simulations even with strong
stellar feedback.
