PCA reconstruction in two and three dimensions: the left most column in each panel shows a sample galaxy in the three maps (metallicity, stellar age, and mass from top to bottom) followed by the low-dimensional PCA representation with 60 (215) eigengalaxies in a) two and b) three dimensions. The rightmost column shows the residual image (I_original − I_rec). You can clearly see that the complex spiral structure in the Stellar Age map is approximated very well in the two-dimensional case while at the same time at n = 60 we smooth the noise in the image. Compare also with Fig. 4 for what happens in the larger n.