A case of TNG50 simulated galaxies with strong age–metallicity break. Top left: temporal evolution of average metallicity of stellar and gas particles, relative vertical velocity dispersion at birth, and gas mass, as shown in orange circles, purple diamonds, green squares, and blue diamonds, respectively. A smoothed age–metallicity relation considering 30% age uncertainty of each stellar particle is presented in gray circles. Black circles and diamonds denote the five mono-age bins or snapshots for further examinations. Top right: surface mass density radial profiles of five mono-age populations of stellar particles. The vertical dashed line marks the half-light radius of 3.3 kpc for this galaxy. Bottom left: radial profile of light-weighted average [Mg/H] of the five mono-age populations of stellar particles. The shaded regions indicate 1σ scatter of the radial profiles. Bottom right: radial profile of mass-weighted average [Mg/H] of the gas particles in the five snapshots around the age–metallicity break. The lookback time of each snapshot is included in the legend.