Relation between mean metallicity and merger time for all the merger debris in the six HESTIA galaxies (indicated by different symbols in the bottom left of the left panel). The symbols are colour-coded according to the debris stellar mass. The left and right panels show the SNd-like and the entire population, respectively. In the right panel, the subplot depicts the difference between the mean metallicity of the entire debris and that of the SNd-like region ((0.5 − 2)R_d and |z|< 10 kpc, where R_d is the disk scale length from Libeskind et al. 2020). Two distributions are shown for the low-mass debris (< 10⁹ M_⊙, blue) and the high-mass debris (> 10⁹ M_⊙, red). The lines of different colours show the linear fits of the mean metallicity-and-merger time relation in several bins of the stellar mass.