Top: density plot of the stellar mass surface density μ_⋆ vs. for the individual sub-kpc regions of M 31. Yellow points indicate a higher density of points, red a lower density. A linear function was fit to the bulge (green line) and disk (blue line) regions. The black line is a theoretical function for pure heating by the old stellar populations. Bottom: same relation, but separated in the macro-regions for M 31: the bulge (red), the inner disk (cyan), the 10 kpc ring (blue), and the outer disk (green). All pixel values are binned in μ_⋆; each point is the average of a bin.