Gaia-Sausage-Enceladus merger timed to Milky Way bar formation via recalibrated Mira variables

The study by Natsuki Funakoshi and colleagues at the University of Tokyo re-anchors 2394 O-rich disc Mira variables from Gaia DR3 proper motions to an action-based quasi-isothermal distribution function. Vertical velocity dispersion is matched to Funakoshi et al. (2025) APOGEE ages derived from a Bayesian neural network trained on Kepler asteroseismology, yielding a unified cosmic clock that avoids tracer mismatches. This timing places the nuclear stellar disc rise and peak squarely within the GSE-induced starburst window, suggesting the merger supplied both fresh gas and inward torques that raised central baryonic density and seeded bar instability.

The coincidence links small-scale stellar kinematics to galaxy-scale morphology: GSE tidal forcing appears to have both contracted the star-forming gas disc and triggered the bar while the thick disc was still assembling. Earlier bar-formation estimates from nuclear-disc Miras and GSE ages from red-giant chemistry could not be compared directly; the single age scale removes that ambiguity and strengthens the causal inference. The result also implies that outer-disc gas driven inward by the merger contributed to the rapid inner-disc growth that stabilized the bar.

Future Gaia data releases with improved proper motions and radial velocities for fainter Miras will test whether the bar age remains locked to the GSE epoch within 0.5 Gyr. Hydrodynamical simulations incorporating a GSE-like merger at 8-10 Gyr should reproduce the observed bar strength and nuclear-disc star-formation history if the proposed seeding mechanism is correct.