Impacts of a flaring star-forming disc and stellar radial mixing on the vertical metallicity gradient. We ran an N-body simulation of the evolution of a barred disk galaxy similar in size to the Milky Way, using our original Tree N-body code, GCD+. We first assumed the thin disk has a constant scale-height, zd, independent of the radii, ”constant zd model”. We then assigned the metallicity of the thin disk particles to follow the radial metallicity gradient of [Fe/H] = 0.5−0.08× R with a dispersion of 0.05 dex with no vertical metallicity gradient at t = 0. Left two panels of Fig. 1 show the initial (at t = 0) and final (t = 8 Gyr) vertical metallicity distribution for the thin disk component of this model (constant zd model). Interestingly, after 8 Gyr of evolution, the vertical metallicity gradient becomes positive, which is inconsistent with the negative vertical metallicity gradients observed in the mono-age population of the thin disk (e.g. Ciucă, Kawata et al. 2018, MNRAS, 475, 1203).
We presented that a flaring thin disk is one possibility to remedy this issue. We made ”flaring model” with a 2nd thin disk component whose vertical scale height increases with radius. This flaring thin disk component mimics a mono-age population that is born in a flaring star-forming region. We assigned the metallicity following the same radial metallicity gradient as our constant zd model to the flaring disk particles. Right two panels of Fig. 1 show the initial (at t = 0) and final (t = 8 Gyr) vertical metallicity distribution for the flaring thin disk component of this model. In this case, the metal poor stars initially in the outer disk become dominant at high vertical height at every radii, which can drive a negative vertical metallicity gradient. Therefore, if mono-age populations of the Milky Way thin disk stars are formed within a flaring star forming disk, each mono-age thin disk population can have a negative vertical metallicity gradient. Then, older population has a steeper negative vertical metallicity gradient, because they have more time to experience more radial mixing. This can explain the steeper negative vertical metallicity gradient observed in the older population of the thin disk with the combined data of Gaia DR1 and RAVE DR5 (Ciucă, Kawata et al. 2018).