Vertical velocity dispersion (σ_Vz) and [α/Fe] as a function of age. (a): σ_Vz as a function of age for the high-[α/Fe] (red) and remaining disc (blue) stars. σ_Vz was measured in bins of 1 Gyr via the bootstrap resampling. The purple triangle represents the high-[α/Fe] stars from Miglio et al. (2021). The brown diamonds represent the velocity dispersion (σ_ext) for the high-redshift galaxies (z ≈ 4.5) from Rizzo et al. (2021). For the redshift-to-lookback-age conversion, we assume h = 0.7, Ω_M = 0.3, and Ω_Λ = 0.7 (Planck Collaboration XIII 2016). (b) Age–[α/Fe] relationship for the full age sample. A KDE is also overplotted to highlight the density features. The colours represent the number of stars per bin in log scale.