Clues to growth and disruption of two neighbouring spiral arms of the Milky Way

Studying the nature of spiral arms is essential for understanding the formation of the intricate disc structure of the Milky Way. The European Space Agency's Gaia mission has provided revolutionary observational data that have uncovered detailed kinematical features of stars in the Milky Way. However, so far the nature of spiral arms continues to remain a mystery. Here, we present that the stellar kinematics traced by the classical Cepheids around the Perseus and Outer spiral arms in the Milky Way show strikingly different kinematical properties from each other: the radial and azimuthal velocities of Cepheids show positive and negative correlations in the Perseus and Outer arms, respectively. We also found that the dynamic spiral arms commonly seen in an N-body/hydrodynamic simulation of a Milky Way-like galaxy can naturally explain the observed kinematic trends. Furthermore, a comparison with such a simulation suggests that the Perseus arm is being disrupted, while the Outer arm is growing. Our findings suggest that two neighbouring spiral arms in distinct evolutionary phases - growing and disrupting phases - coexist in the Milky Way.