Fig. 3: Correlation between the integrated stellar metallicity gradients normalized to the galaxy effective radius and the disk structure evolution.
a,b, The disk structure evolution is quantified as the difference between the slope of radial luminosity surface density profiles (Δ log(ΣL)/ΔR) of young (0–4 Gyr) and old (8–12 Gyr) stellar populations, inside (a) and outside (b) the break radius identified in the metallicity radial profiles. Filled circles denote Milky Way-mass, star-forming galaxies in the TNG50 simulation, colour-coded by their effective radius. Two insets on the left-hand side of each panel illustrate the luminosity surface density profiles of young and old populations in two example TNG50 galaxies (black squares): one with strong evolution at the top and one with nearly parallel growth (a) or inverse evolution (young population being more compact, b) at the bottom. The possible position of the Milky Way in these two diagrams is denoted by a shaded ellipse, which is stretched by the large uncertainty on the Milky Way’s effective radius, due in turn to uncertainties in the disk scale length measurement and to the possible deviation from a single-exponential density profile. We adopt the same range of effective radius of our Galaxy as in Fig. 2 and assume equally likely values within this range.
