2025ApJ...986..123H

The Milky Way is a dynamic and evolving system shaped by numerous merger events throughout its history. These mergers bring stars with kinematic and dynamic properties differing from the main stellar population. However, it remains uncertain whether any of the Galactic supernova remnants can be attributed to such a merger origin. In this work, we compare the progenitor of Kepler's supernova to its nearby stars, “alien” stars, and in situ Milky Way stellar populations. We uncover the abnormal kinematics and dynamics of Kepler's supernova and propose that its progenitor may have an extragalactic origin. We call the Type Ia supernovae (SNe Ia) produced by stars accreted into the Milky Way through merger events “alien SNe Ia” since they are cosmic immigrants. We estimate the rate of alien SNe Ia exploded recently using two methods: through galactic chemical evolution, and through a method without considering exact star formation history, introduced for the first time in this paper. We consider the past accretion of a few major satellite galaxies-Kraken, Gaia-Enceladus-Sausage, the Helmi streams, Sequoia, Sagittarius, Wukong/LMS-1, and Cetus-assuming these were dry mergers. The first method yields 1.5 × 10^–5-5.0 × 10^–5 yr^–1, while the second method yields a comparable ${3.1}_{-1.1}^{+1.8}\times 1{0}^{-5}\,{\mathrm{yr}}^{-1}$ as the rate estimates for recent alien SNe Ia. These estimates represent lower bounds because we assumed no postmerger star formation.