Structure of different models after 0.6 Gyr: M1 (no initial gradient), M2 (both ISM enrichment and gradient) and M3 (only gradient). From left to right: stellar density, gas density, the mean ISM metallicity, residual ISM metallicity (after subtraction of the radial gradient), and the radial metallicity profile. Different models are shown in different rows. Coloured lines highlight the location of the stellar spiral arms measured as a positive overdensity in the first row (see Sect. 3 for details). In the right column, the mean radial metallicity trend is shown by the white solid line. The exponential fit of the radial gradient is shown by the red lines (shifted vertically for better visibility), where R₂₅ = 12 kpc.