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

TL;DR

This study reports the discovery of very young star clusters in the distant outer galaxy, likely triggered by an impact from a high-velocity cloud, revealing a new star formation mechanism in extreme galactic environments.

Contribution

It provides the first evidence of star formation in the most distant outer galaxy possibly induced by high-velocity cloud impact, using deep infrared imaging and kinematic analysis.

Findings

Two young embedded clusters identified at 16 kpc distance.

Clusters are less than 1 million years old, indicating recent star formation.

Star formation likely triggered by interaction with high-velocity cloud Complex H.

Abstract

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.