Hβ and [MgFe]′ indices (left panel) and Mgb and ⟨ Fe ⟩ indices (right panel) measured along the major axis of NGC 4138. The red triangles and blue squares correspond to the values measured at | r | ≃ 21″ for the co-rotating and counter-rotating components, respectively. The gray filled circles are the line strengths measured in the inner 12″ where only the main component is present. The center of the galaxy is indicated by an open gray circle. The lines indicate the models by Thomas et al. (2003). In the left panel the age-metallicity grids are plotted with two different α/Fe enhancements: [α/ Fe] = 0.0 dex (continuous lines) and [α/ Fe] = 0.5 dex (dashed lines). In the right panel the [α/ Fe] ratio-metallicity grids are plotted with two different ages: 2 Gyr (continuous lines) and 12 Gyr (dashed lines).
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