Figure 2. Cumulative age distributions for RC stars at different galactocentric radii. The shaded areas represent for each distribution the 1σ range from 1000 bootstrap samples. The top panel shows stars in the geometrically defined thick disk (i.e., stars far from the midplane), while the bottom panel shows stars in the chemically defined thick disk (i.e., α-rich stars). The α-rich population is quite uniform as a function of galactocentric radius, with an age distribution consistent with a 2.5 Gyr-wide Gaussian centered on 8 Gyr (black line). By contrast, the age distribution of the geometrically defined thick disk changes significantly as a function of radius, with younger stars in the outer regions.
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