Ratios of the number of FGK stars hosting Earth-like planets (N_★,mHC) in models with stellar migration to the reference model without migration, as a function of Galactocentric distance and Galactic time. All considered models adopt case 2 for SN damage. Upper panel: stellar migration model with σ_RM = 3.5 (ratio of model 3 to model 1 in Table 1). Lower panel : stellar migration model with σ_RM = 6 (ratio of model 4 to model 1 in Table 1).