Debris of Gaia-Sausage-Enceladus that made a H I hole in the Milky Way ≈20 million years ago

The Perseus arm is known as one of the two or four dominant spiral arms of the Milky Way. While there is a large number of Massive Young Stellar Objects in the outer portion of the arm, a lower density of those is found in the inner portion. Inner Perseus arm shows a noncircular motion of ≥ 70 km/s at a Galactic longitude of ≈50◦, and its origin remains unclear. Here we report an analysis of the kinematics and spatial distribution of neutral hydrogen (H I) gas, star-forming regions (SFRs) and stars, together with an analysis of the star’s chemical abundances. We discovered that H I gas with ≈10^6 solar mass was lacked in the inner Perseus arm, and a similar amount of H I gas was distributed above the Galactic plane. The extended H I gas is well followed by retrograde low-metallicity stars, which are likely fossil stars from Gaia-Sausage-Enceladus. Orbit integration shows that the fossil stars crossed the inner Galactic disk about 20 million years ago. The lower star-formation activity and noncircular motion of the inner Perseus arm could be attributed to the disk crossing event.