Impacts of a Flaring Star-forming Disc and Stellar Radial Mixing on the
  Vertical Metallicity Gradient
  Using idealised N-body simulations of a Milky Way-sized disc galaxy, we
qualitatively study how the metallicity distributions of the thin disc star
particles are modified by the formation of the bar and spiral arm structures.
The thin disc in our numerical experiments initially has a tight negative
radial metallicity gradient and a constant vertical scale-height. We show that
the radial mixing of stars drives a positive vertical metallicity gradient in
the thin disc. On the other hand, if the initial thin disc is flared, with
vertical scale-height increasing with galactocentric radius, the metal poor
stars originally in the outer disc become dominant in regions above the disc
plane at every radii. This process can drive a negative vertical metallicity
gradient, which is consistent with the current observed trend. This model
mimics a scenario where the star-forming thin disc was flared in the outer
region at earlier epochs. Our numerical experiment with an initial flared disc
predicts that the negative vertical metallicity gradient of the mono-age
relatively young thin disc population should be steeper in the inner disc, and
the radial metallicity gradient of the mono-age population should be shallower
at greater heights above the disc plane. We also predict that the metallicity
distribution function of mono-age young thin disc populations above the disc
plane would be more positively skewed in the inner disc compared to the outer
disc.
