Abundance ratios of [Si/Fe] as a function of [M/H] predicted by the three-infall chemical evolution model presented in Sect. 3 and with parameters listed in Table 1. Different colours indicate the distinct infall episodes: red (high-α), dark blue (low-α, part I), and light-blue (low-α, part II). Filled yellow circles indicate the abundance ratios of the chemical evolution model at the given age. In the inset, the normalised star formation history is reported with the different evolutionary phases coloured, as in main plot [Si/Fe] versus [M/H]. The vertical dashed lines indicate the ages corresponding to yellow points.