Star formation in the circumgalactic high-velocity cloud Complex H

TL;DR

This study discovers a young, metal-poor star cluster within the high-velocity cloud Complex H, demonstrating that such clouds can form stars and contribute to galactic accretion processes.

Contribution

It provides the first direct evidence of star formation in an HVC, linking stellar properties to cloud dynamics and origin, and suggests HVCs can produce high-velocity stars.

Findings

Identified a binary open cluster in Complex H with age ~11 Myr.

Measured the cluster's metallicity at about 5% of solar.

Demonstrated the cloud's orbit and interaction with the Galactic disk.

Abstract

The accretion of metal-poor gas sustains galactic star formation. In the Milky Way, this process is fueled by high-velocity clouds (HVCs), yet their fundamental properties have remained elusive in the absence of stellar tracers. Here we report a binary open cluster within HVC Complex H. With an age of 11.2 +- 0.6 Myr and a subsolar metallicity of 0.05(+0.05-0.02) Zsun, the clusters provide a direct stellar distance anchor to the cloud at 13.8 +- 0.6 kpc. Their proper motions indicate Complex H is on a prograde, south-to-north orbit through the outer Galactic disk. The resulting interaction produces a 'slow-fast-slow' velocity gradient, with the cloud's outer layers decelerating as they merge into the disk. Orbit integration suggests the clusters formed from an internal cloud-cloud collision. This triggering mechanism implies other HVCs could similarly produce high-velocity stars. The scarcity of previous stellar detections in HVCs is explained by the rapid escape of young stars (< 20 Myr), while CO non-detections may stem from weak emission due to low metallicity and gas dispersal. This work reveals that the circumgalactic medium can sustain star formation, offering a tangible laboratory to probe the physical conditions of accreting gas before it merges with the Galactic disk.