Spectral index of the thick disc plotted against different galaxy properties. The panels show the spectral index versus total mass (Makarov et al. 2014), star forming radius (Wiegert et al. 2015), SFR surface density within r_★ (Heesen et al. 2025), total mass surface density scaled to our distances (Irwin et al. 2012), ratio of SFR-to-mass surface density, and SFR from Hα and mid-infrared (Vargas et al. 2019). Data points represent the galaxies in our sample with their associated uncertainties.