The impact of radial migration on disk galaxy star formation histories

Knowledge of the spatially resolved star formation history (SFH) of disk galaxies provides crucial insight into disk assembly, quenching, and chemical evolution. However, most reconstructions, both for the Milky Way and for external galaxies, implicitly assume that stars formed at their present-day radii. Using a range of zoom-in cosmological simulations, we show that stellar radial migration introduces strong and systematic biases in such SFH estimates, and in a Milky Way–like case study we link these biases directly to the disk’s merger-driven, non-axisymmetric response. In the inner disk (R ≲ hd), early star formation is typically underestimated by 25–50% and late star formation overestimated, giving the misleading impression of prolonged, moderate activity. An exception occurs in the most central bin considered (∼0.4hd), which is consistently overestimated due to a net inflow of inward migrators. At intermediate radii and in the outer disk, migration drives the opposite trend: intermediate-age populations are overestimated by 100–200% as stars born in the inner disk migrate outward, whereas genuinely in situ populations are underestimated by ∼50% as they themselves continue to migrate. The net effect is that SFH peaks are suppressed and broadened, and the true rate of inside-out disk growth is systematically underestimated. These distortions affect all galaxies in our sample and have direct implications for interpreting spatially resolved SFHs from integral field unit surveys such as CALIFA and MaNGA, where present-day radii are often used as proxies for stellar birth sites. Correcting these biases will require accounting for the disk mass, bar presence, disk kinematics and morphology, and recent birth-radius estimation techniques for Milky Way stars offer a promising path forward.