The stellar mass surface density − age relationship resulting from fitting the 98291 spectra of 107 galaxies. The color bar shows the density of spectra per plotted point (red-orange are a few tens of spectra). Also plotted (as larger circles) are the averaged values for each galaxy, obtained as explained in Sect. 6. The colors of these circles code the galaxy mass (orange-red are galaxies more massive than 10¹¹ M_⊙); the dashed line marks μ_∗ = 7 × 10² M_⊙/pc².