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Fig. 4.


Lick/IDS indices measured from our spectra of the UDG (blue) and the stacked GC spectrum (red). The cloud of 100 points (orange) shows the bootstrap runs of the GCs, as detailed in the text, and indicates the scatter intrinsic to the stack. The underlying model grid is based on Thomas et al. (2010). A grid with ages of 1, 3, 5, 7, 10, and 13 Gyr (dashed lines), and [Z/H] of −2.25, −1.35, −0.33, 0.0, +0.35, and +0.67 dex (dotted lines) is overplotted in both panels. For clarity, lines for 1 and 3 Gyr are omitted in the left panel. Left panel: comparison of Mg b and ⟨Fe⟩ as a measure of α-enrichment. Right panel: comparison of [MgFe]′, a measure of total metallicity, and Hβ, which is primarily an age indicator.

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