Discovery of Star Formation in the Extreme Outer Galaxy Possibly Induced by a High-velocity Cloud Impact

We report the discovery of star formation activity in perhaps the most distant molecular cloud in the extreme outer galaxy. We performed deep near infrared imaging with the Subaru 8.2 m telescope, and found two young embedded clusters at two CO peaks of Digel Cloud 1 at the kinematic distance of D = 16 kpc (Galactocentric radius RG = 22 kpc). We identified 18 and 45 cluster members in the two peaks, and the estimated stellar density are ~ 5 and ~ 3 pc^-2, respectively. The observed K-band luminosity function suggests that the age of the clusters is less than 1 Myr and also the distance to the clusters is consistent with the kinematic distance. On the sky, Cloud 1 is located very close to the H I peak of high-velocity cloud (HVC) Complex H, and there are some H I intermediate velocity structures between the Complex H and the Galactic disk, which could indicate an interaction between them. We suggest possibility that Complex H impacting on the Galactic disk has triggered star formation in Cloud 1 as well as the formation of Cloud 1 molecular cloud.