Fig. 9: Age dependence of the radial abundance gradient (∂[Fe/H]/∂R) and the corresponding abundance dispersion (σ_[Fe/H]) around the gradient.

The uncertainties (error bars) in each panel are derived using MCMC (Markov Chain Monte Carlo) fitting, employing the Python code emcee⁶⁹. a Age dependence of the radial [Fe/H] (∂[Fe/H]/∂R_guide, black solid line) gradient and radial [O/Fe] (∂[O/Fe]/∂R_guide, blue solid line) gradient, in terms of guiding-centre radius R_guide. Each point was obtained by 3-parameter (slope, intercept, and dispersion) Bayesian fits to the [Fe/H]/[O/Fe]-R_guide distribution, using only data in the respective age bin. The black and blue dashed lines represent the radial [Fe/H]/[O/Fe] gradients (∂[Fe/H]/∂R_birth and ∂[O/Fe]/∂R_birth) in terms of birth radius R_birth, calculated for the same age bins. The difference between the dashed lines and the solid lines can be attributed the influence of radial migration (churning). b Age dependence of the [Fe/H] (\({\sigma }_{{{{\rm{[Fe/H]}}}},{{{{\rm{R}}}}}_{{{{\rm{guide}}}}}}\), black) and [O/Fe] (\({\sigma }_{{{{\rm{[O/Fe]}}}},{{{{\rm{R}}}}}_{{{{\rm{guide}}}}}}\), blue) dispersion around the radial [Fe/H]/[O/Fe] gradient (a), in terms of R_guide. The grey-shaded area marks the age interval in which we expect to see signatures from the Gaia Sausage/Enceladus (GSE) merger event, while the red-shaded area marks the age interval for the effect of Sagittarius dwarf galaxy (Sgr) passage. Source data are provided as a Source Data file.