Star formation history of the inner Milky Way (R < 10 kpc) derived from fitting the solar vicinity age-[Si/Fe] abundance with a chemical evolution model in Snaith et al. (2014; 2015; orange curve, left axis), together with the gas fraction evolution in the model (thin black curve and right axis). The SFR at ~8 Gyr is negligible (consistent with no star formation), while the gas fraction is still very high in the system, similar to the molecular gas fractions estimated in disks at redshifts ~1−3 (Daddi et al. 2010; Arevana et al. 2010; Dannerbauer et al. 2009; Tacconi et al. 2010, 2013). The SFH is normalised such that the current stellar mass of the Milky Way is 5 × 10¹⁰M_⊙.