Impact of the IMF on [Mg/Fe] and Mgb measurments. Left panel: [Mg/Fe]–σ relation of our stack spectra using FIF (orange). The empty circles represent the results obtained with a fixed Milky Way-like IMF, and the filled circles show our measurements when the IMF is treated as an additional free parameter. The black lines correspond to literature [Mg/Fe] measurements: Johansson et al. (2012) (solid line), Worthey et al. (2014) (dotted line with data points), Conroy et al. (2014) (dashed line with data points), Thomas et al. (2005) (dash-dotted line), and McDermid et al. (2015) (dashed line). Right panel: Mgb–σ relation of our stack spectra and the relative contribution to the Mgb strength from age, metallicity, [Mg/Fe], and IMF variations.