NIR Mg and Si absorption line strengths in NGC 1277 and massive ETGs. Top: zoomed-in view of the spectra of NGC 1277 (pink line) and the stacked massive ETGs (cyan) around the MgI1.18, SiI1.20, and SiI1.59 features, with their corresponding error spectra. Yellow bands highlight these key absorption features, and light blue regions show the pseudo-continuum bands used for their index measurements. As reference, we show in grey an E-MILES model spectrum for an old, metal-rich, dwarf-enhanced population. Bottom: measured line-strength indices of MgI1.18, SiI1.20, and SiI1.59 plotted as a function of stellar population ages. Model predictions are shown for E-MILES assuming different IMF shapes (Milky Way-like in black, bottom-heavy in grey) and metallicities (solar as solid lines, super-solar as dotted lines). Arrows show index responses to +0.3 dex enhancements in [Mg/Fe] (blue) and [Si/Fe] (orange) using response functions from Conroy et al. (2018), attached to models representing the average stellar population of massive galaxy centres. The observed values are overlaid as pink circles (NGC 1277) and cyan diamonds (local massive ETGs). The Mg enhancement in NGC 1277 confirms previous results in the optical, while the stronger Si absorption (∼25% higher than in typical ETGs) cannot be matched by current models, even when varying IMF or metallicity, suggesting a distinct chemical enrichment.