High-Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?

Observations with the Green Bank Telescope of 21 cm H I emission from the high-velocity cloud complex H suggest that it is interacting with the Milky Way. A model in which the cloud is a satellite of the Galaxy in an inclined, retrograde circular orbit reproduces both the cloud's average velocity and its velocity gradient with latitude. The model places complex H at R = 33 ± 9 kpc from the Galactic center on a retrograde orbit inclined ≈45° to the Galactic plane. At this location, it has a mass in H I of greater than 6 × 106 M☉ and dimensions of at least 10 × 5 kpc. Some of the diffuse H I associated with the cloud has apparently been decelerated by interaction with Galactic gas. Complex H has similarities to the dwarf irregular galaxy Leo A and to some compact high-velocity clouds, and it has an internal structure nearly identical to parts of the Magellanic Stream, with a pressure P/k ≈ 100 cm-3 K.