Gas circularity parameter-radial distance-stellar metallicity diagrams at four different times. At epoch &#Xtextcircled;b, the pre-existing disk (ϵ_* > 0) has a high metallicity (log(Z/Z_⊙) >  0) compared to the accreted gas (log(Z/Z_⊙) ∼ –0.7). We note that at r ∼ 1 kpc, there is some gas mixing at the interface of the two disks. At epoch &#Xtextcircled;c, the accreted gas has totally replaced the pre-existing one. Due to the lack of star formation and therefore to a subsequent low supernovae activity, the new disk tend to keep its original metallicity. In epoch &#Xtextcircled;d, the same phenomenon repeats: coexistence of two disks. The accreted gas is characterized here by a very low metalliciy (log(Z/Z_⊙) <   − 1). Then in epoch &#Xtextcircled;e, the new disk has totally replaced the older one, but due to a star formation activity slightly more pronounced during the gas accretion, the new gas disk has a metallicity that gradually increases from smaller to larger radii.