Impact of Gaia-Sausage-Enceladus merger revealed by a single cosmic clock: thick-to-thin disc transition, gas disc shrinking and bar formation

Natsuki Funakoshi; Daisuke Kawata; Jason L. Sanders; Ioana Ciucă; Robert J. J. Grand; Noriyuki Matsunaga; Brian Thorsbro; HanYuan Zhang

arXiv · 2610.30237

Impact of Gaia-Sausage-Enceladus merger revealed by a single cosmic clock: thick-to-thin disc transition, gas disc shrinking and bar formation

Natsuki Funakoshi·Daisuke Kawata·Jason L. Sanders·Ioana Ciucă·Robert J. J. Grand·Noriyuki Matsunaga·Brian Thorsbro·HanYuan Zhang

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Abstract

Whether the formation of the Galactic bar was connected to the Gaia-Sausage-Enceladus (GSE) merger, the last significant merger experienced by the Milky Way, remains an open question. The bar formation epoch has been inferred from the star formation history of the nuclear stellar disc (NSD) traced by Mira variables, and the GSE merger epoch from the age-metallicity relation of red giants. However, these epochs rest on different stellar tracers with different age scales and cannot be compared directly. We recalibrate the Mira period-age relation onto the age scale of APOGEE red giants, with ages from a Bayesian neural network trained on Kepler asteroseismic ages. We fit the Gaia DR3 proper motions of 2394 O-rich disc Mira variables with an action-based quasi-isothermal distribution function whose parameters vary with period, and derive the period-age relation by matching their vertical velocity dispersion as a function of period to that of the APOGEE red giants as a function of age in Funakoshi et al. (2025). This places the NSD star-formation rise and peak at 8.5-10.5 Gyr ago on the red giant age scale, overlapping with the final stage of the GSE-induced starburst, thick-to-thin disc transition and the contraction of the star-forming gas disc. If NSD star formation traces bar formation, these coincidences suggest a possible scenario in which the GSE seeded the bar formation, while fresh gas supplied by the GSE and outer disc gas driven inward by its tidal torques, increased the baryonic dominance of the inner Galaxy.

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Natsuki Funakoshi, Daisuke Kawata, Jason L. Sanders, Ioana Ciucă, Robert J. J. Grand, Noriyuki Matsunaga, Brian Thorsbro, HanYuan Zhang. 2026-10-21. Impact of Gaia-Sausage-Enceladus merger revealed by a single cosmic clock: thick-to-thin disc transition, gas disc shrinking and bar formation. https://arxiv.org/abs/2610.30237

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